Mind Flay: Insanity - Shadow Priest Theorycraft from a Practical Standpoint

Friday, September 12, 2014

Mind Searing - Stop to Mind Blast?

It's been a while since I've posted! I was going to wait until WoD to do more posts, but recently several people have been wondering what to do about AoE. Specifically, they're wondering if they should interrupt Mind Sear to cast Mind Blast. Some guides say that orb generation is always the most important thing no matter what, while others don't even mention Mind Blast in the AoE rotation. The confusion is real, and it's time to put some people at ease in this situation. The road is bumpy, though. How long will the adds live? How many of them are there? These things are critical to know. If they will die quickly, is it really worth Mind Blasting right this second and missing out on a big portion of a Mind Sear opportunity, since they can be few and far between? After all, Mind Blast will be there for us to use when all of the adds are dead, but we may only get to Mind Sear once or twice in a fight. If we have 20 targets to Mind Sear instead of only 4, does that change anything? Is it better to keep Mind Searing, or should we stop to cast Mind Blast? As usual, it depends.

In order to compare continuing with Mind Sear to stopping for a Mind Blast, we need to figure out exactly how much damage Mind Blast is doing for us. It hits hard, sure, but it also gives us a shadow orb, so we'll add in the value of an orb, 1/3 of a DP, and also add the value we'd get from Insanity from that 1/3 DP (if we're talented for it, which we almost always are currently). Then, see how many targets it would take for Mind Sear to be better dps than stopping to Mind Blast. Although in T16 we use Insanity for most of the fights, I'm going to break this down separately for when you do and do not have Insanity talented. Insanity adds extra value to shadow orbs, and since the value of shadow orbs is important to what we're looking at, this distinction is necessary.

Here's the tricky thing, though. This assumes you're missing an entire Mind Blast and all of the extra damage that comes with it by continuing to Mind Sear. Are we really? If you Mind Sear for 5 seconds after MB comes off cooldown, Mind Blast is still there to use afterwards. What we did was delay it by 6/9ths (5s late + 1s cast time) of its usual interval (8s cd + 1s cast time = 9s). So really we lost 6/9ths of a Mind Blast and everything associated with it. To take the delay of Mind Blast rather than complete loss of it into account, we need to multiply the total MB damage by (t+1)/9, where t is the length of time you expect to be Mind Searing while MB is ready to be used.

With Insanity Talented:

Mind Sear:
This damage begins at 315 + 0.3*SP every 1 sec for 5 sec. Multiply that by 1.5 because a Mind Blast takes 1.5s to cast (both scale with haste, so if you're like me and have a 0.66s tick interval for Mind Sear, you also have a 1.0s Mind Blast cast time, which is again a 1:1.5 ratio) and we get the..
Total for Mind Sear: (472.5 + .45*SP)*numTargets, where numTargets is the number of targets being hit by Mind Sear

Now we'll take MB, 1/3 of a DP assuming you use it at 3 orbs, and 3 ticks of Insanity's increase in damage to a Mind Flay (assuming we'd get 9 ticks of Insanity from the DP, 3 ticks is 1/3 of that Insanity bonus) to get the total damage that actually results from a MB. Subtract 1/2 a Mind Flay tick to compensate for 1/3 of a GCD that you spend using the orb you get from this Mind Blast, too.
Mind Blast: 2768.5 + 1.909*SP
1/3 of a DP @ 3 orbs: 3286 + 1.572*SP, but haste gives us extra ticks at break points, so we need to add those in. At and beyond 14,873 haste, we have +3 ticks of DP. Many of us are at that point, so I'll assume this number and add in 3*(274 + 0.131*SP) for 3 more ticks of 1/3 of a 3 orb DP, which gives us a total of 4109 + 1.965*SP.
3 ticks of Insanity bonus: 3147 + 1.5*SP (note that this is only the extra damage to Mind Flay added by Insanity, since if we didn't DP we'd be doing regular Mind Flay, so we subtract that damage out to get the increase given from the Mind Blast's orb)
-1/2 of a Mind Flay tick: -524.5 - 0.25*SP
Total from Mind Blast: (mbDelay+1/9)*(9500 + 5.12*SP), where mbDelay is the time using Mind Blast will be delayed after it's off cooldown.

Since we're interested in the difference between these two values, we simply subtract Mind Sear's damage from Mind Blast's damage. You'll notice that SP doesn't cancel out, which means Spell Power matters. You'll also notice that Mind Blast's SP multiplier is quite a bit higher until we have 12 targets to Mind Sear, so the more spell power you have the stronger MB is compared to Mind Sear until you reach 12 targets. In order to reduce a variable for now, I'm going to be looking at 50,000 Spell Power. If you don't have that much or have even more, don't worry. It doesn't change that much, but feel free to calculate it out for your own levels if you're way below 50k. :)

So now we're factoring in the number of adds and how long we're going to be delaying Mind Blast. This feels like graph time to me. To make your own, simply set the above two totals equal and then solve for numTargets or mbDelay, then plug in relevant possibilities for those variables and fill a table with your results.

Well, there it is. You'll want a rough estimate of how many seconds you'll be delaying Mind Blast. Then, get a good idea of how many things you will be Mind Searing. A good take-away from this is that if there are 11 targets (think Immerseus AoE), you can just continue happily Mind Searing forever. If there are fewer than that, say 5, and you think they'll be around for a while, then keep Mind Blast on cooldown. If they'll be dying pretty quickly (3s or less), go ahead and delay Mind Blast to burst them down really quickly. As another example, on Garrosh phase 1, we get 7 targets (Garrosh + 6 melee adds). The first set dies very quickly in my guild, so I won't be interrupting for Mind Blast. The later sets tend to last a bit more than 4.5 seconds, so I'm better off interrupting MS for a quick MB.

Without Insanity:

This part is much easier to figure out. We're simply comparing Mind Sear to Mind Blast plus 1/3 of a DP minus 1/3 of a GCD of Mind Flay filler.

From the above, nothing changes for Mind Sear.
Total for Mind Sear: (472.5 + .45*SP)*numTargets, where numTargets is the number of targets being hit by Mind Sear

On the other side, we'll take Mind Blast, add in 1/3 of a 3-orb DP, and subtract out 1/3 of a GCD for that DP. We'll again use Mind Flay filler for that GCD.
Mind Blast: 2768.5 + 1.909*SP
1/3 of a DP @ 3 orbs, +3 ticks haste break point: 4109 + 1.965*SP
-1/2 of a Mind Flay tick: -524.5 - 0.25*SP
Total for Mind Blast: (mbDelay+1/9)*(6353 + 3.62*SP), where mbDelay is the time Mind Blast will be delay after it's off cooldown.

And of course we're going to look at the difference between these two things in relation to both the number of targets and the time you expect to be delaying Mind Blast, so it's time for another graph! This one also assumes 50,000 spell power.

So, if you're not talented for Insanity, delaying Mind Blast becomes a better option more often. You can optimally spam Mind Sear forever and ignore orbs if there are 8 targets that will be hit by it. Not having the extra damage from Insanity baked into Mind Blast makes a noticeable difference!


First, this post hasn't mentioned procs. Going into detail to cover them would make this post insanely long. Briefly, procs tend to favor Mind Searing. Much of the damage baked into Mind Blast comes later on in the form of DP and Insanity, and they likely won't be benefiting from the procs. I also assumed the +3 DP haste break point. If you don't have that much haste, then the total MB damage is overvalued for you slightly. Most people should have at least +2, though, which isn't hugely different. I also assumed 50,000 Spell Power. Spell Power definitely affects the out, so if you have 20k SP, you'll want to recalculate things. The difference isn't drastic for anywhere near but not exactly at 50k SP, though. If you're in mostly T16 normal or better gear, this is easily close enough. Also, if you're spec'd for DI, the interval calculation isn't always going to be accurate. You'd lose a lot more dps by having a DI proc get refreshed by another one during Mind Sear, though, so go ahead and use procs during AoE. Finally, this post assumes you'll actually use Insanity well and aren't orb capped. If you won't be able to use Insanity because there's lots of stuff to Mind Sear, and Sear will be better than Insanity due to the number of targets, then treat it as though you don't have Insanity talented.

I hope that will help clear things up when it comes to an aspect of AoE that has not been fully covered in guides before! Please feel free to send me questions about this or any other aspect of playing a Shadow Priest!

Tuesday, November 12, 2013

DP and the BBoY trinket from Garrosh

A lot of smart people have been wondering how exactly to handle using the Black Blood of Y'Shaarj, particularly when you're spec'd into Solace and Insanity. If you've seen this site's url, you can bet that on most fights I've chosen this talent, so I've also been wanting to know how exactly the BBoY stacks fit into our rotation with Devouring Plague and MF:I. There's a reason why everybody just kind of guesses or goes on some vague intuition: it's actually an incredibly intricate matter when you start trying to figure it out. I wanted to find out as best I can what the best way to handle this situation is. I've covered recently that it's possible to use DP with only 2 orbs, and this trinket provides circumstances where that's favorable, but this post is going to focus on the situation where we're getting our 3rd orb proximal to the BBoY proc'ing and also where we have MF:I talented. If you're not using MF:I, the answer is simple: just hold DP for higher stacks. With MF:I, it's more complicated. Do we hold onto our orbs and cast an absolutely crushing DP at 9-10 stacks? Do we cast DP early on and try to get as much insanity action as we can with the higher stacks of the proc? When do we refresh our VT/SWP so they benefit from the proc, too?

In general, you want to wait until 5-6 stacks, refresh SWP/VT, then hit DP and cast as much MF:I as possible.


I'm going to make comparisons by simply going through the 10 seconds, choosing different spells to cast, and multiplying their spell power coefficients by the spell power given by however many stacks of the BBoY proc are present at the time. I'm also including the damage constant. The better choices will have higher total (damage constant + SP coefficients * current spell power from the proc) during these 10 seconds.

For instance, if we get our 3rd orb right as BBoY procs and has 1 stack, we can cast DP right away, channel as much MF:I as we can, and then refresh SWP and VT. In this case, we may not have even noticed the trinket proc yet and have already cast DP, but we'll still use this case in the comparison. We need to find out more specifically the numbers we're dealing with, such as exactly how many ticks of MF:I we're be seeing. In this case, we may already know that we can get as many as 11 ticks of insanity, but for most other cases, we'll also be channeling MF for a few ticks here and there, and so I came up with a time line graph to keep track of what's being cast when and how many stacks the trinket is at. It turned out to be a pretty big monster, and I even left out some of the possibilities once other combinations made certain patterns clear.

(Click to Enlarge)

This gives a pretty good idea of what we're cooking with. The idea was basically just to make sure the right number of MF and MF:I ticks are used in the different areas between refreshing dots or using mind blast and to see how many stacks of BBoY we're getting for each spell we cast. You'll notice that most of the iterations have MB at the far left. To keep things even, I assumed for most of the options that you just cast Mind Blast right as the proc happened. If you cast your 3rd Mind Blast earlier, then you probably already dumped your orbs into Devouring Plague and the whole discussion is moot. Therefore MB is included in each case at least once. Where exactly it is in many of the cases isn't a big deal and the overall findings hold true if it were to be moved around and compared to other possibilities with similar changes to MB.

After I tallied up all of the ticks and kept track of how many stacks the trinket was at for each cast, I simply plugged those into a spreadsheet that multiplies the SP coefficient of the spell by the spell power granted by the trinket at the time of the cast (and included the static damage portion of the spell as well). My numbers are all done using a 561 BBoY, but you can do the same with any of them. Make sure that you multiply the int proc by 1.05 (for 5% stats buff) and by 1.10 (for the 10% spell power buff) to get the total spell power you're getting.

Anyway, here's what I came up with for each of the possibilities on the time line:

* "DP at X w/ 2nd MB" above does not include 4pc t16. Using another MB after DP during the BBoY is a great choice if you have the 4pc t16. *

The explanations of these in the chart may be next to useless, but each case here correlates directly to one in the timeline graph above, so look at that if you want to know exactly what's going on in each case. A few trends stuck out that helped to eliminate some combinations. Yeah, this wasn't all of them. First, using DP early on is basically always bad. DP and MF:I are just too powerful to miss out on the massive spell power gains from the trinket. This is even more true for DP than for insanity. Another theme present is that being able to snapshot a high stack of the trinket with one last MF:I cast is great. You want to at least delay DP long enough that you'll be able to do this. Refreshing your VT and SWP is a good way to spend some the time as you're waiting to get to 7-9 stacks for your DP. It tends to be better to refresh them first, waiting for medium-sized stacks, and use the very highest stacks for DP/MF:I. This surprised me, as I expected stopping MF:I to cast them at 9-10 stacks would be pretty awesome. It still is, but there are even better ways to handle the trinket. Lastly, recasting your Mind Blast after DP isn't that bad. In the bigger picture, it's going to be even better than this chart shows because it's not looking at the delay of your orb generation. Also, Tanned pointed out to me that the 4pc T16 will be affecting this MB. If you have 4pc T16, it's best to use your next MB during the BBoY proc. Thanks, Tanned!

Well, there you go. I hope that this can put some minds at ease. Please send me a message or leave a comment to let me know what you think, and always send me more questions and ideas!

Monday, October 21, 2013

Devouring Plague: 1, 2 or 3 Orbs?

We all <3 Devouring Plague, so I want to see if we can use <3 orbs with it. The rule of thumb for Shadow Priests is that you wait until you have 3 orbs before you use Devouring Plague. This is a good rule to start off with. DP's damage increases linearly with the number of orbs you use, so the main thing you're doing by casting it with fewer orbs is casting it needlessly more often, which wastes globals that could be spent channeling our filler, Mind Flay. However, we know from Refreshing Dots for Procs that, if we consider wasted globals to be a loss of mind flay damage, we can sometimes find ways to use those GCDs better than just spamming MF. We're constantly striving to do anything better than Mind Flay, and if we're observant, such as paying attention to what procs we have active, that's often possible. Casting Devouring Plague with fewer than 3 orbs will yield an increase in DPS depending on the procs you have active. How many orbs? How many procs? Let's find out.

As I mentioned, DP (and MF:I) increases linearly with the number of orbs used. GCDs aside, casting DP every time you have one orb is just as good as waiting for 3 orbs every time. Knowing this, let's compare those gcds, in the form of mind flay damage, to the gain we get from casting DP when we have strong procs. After all, it makes sense that if we can cast DP under lots of procs, we'll get more damage out of it. Waiting for 3 orbs and then having to cast it without any procs at all makes those Siege of Orgrimmar bosses laugh at us. Another thing that's essential to take into account here is the difference between haste procs and other procs. DP scales up with haste by adding a tick at certain breakpoints. With other procs, it scales smoothy, but with haste it's a stepped gain rather than a smooth one, and so it needs to be considered differently. If you have any questions or concerns about the following math, feel free to ask! I make mistakes, and a major goal in writing this is to know for myself the correct answers to these questions, too.

As always, you should be using Affdotspriest by the amazing Alae. It allows us to see how strong our procs are in a way that is quick enough to make solid decisions even under the pressure of a raid boss fight.

Non-haste procs comparison:
Casting DP with 1 orb instead of 3 means we're using 2 more GCDs than usual on it (3 casts for every 3 orbs instead of 1 cast). Casting DP with 2 orbs instead of 3 means we're using 1 more GCD than usual on it. We need to compare some buffed MF damage to the increase in DP from the buffs.

Where 'S' is spell power and 'n' is the number of orbs used for DP @ +3 DP breakpoint (14,873 haste),

The formula for Mind Flay damage is: 3147 + 1.5*S
The formula for Devouring Plague damage is: n(1643 + 0.786*S + 9(274 + 0.131*S))

When using 1 orb, we "waste" 2 gcds, which is 1 MF channel. This gives us: d*MF = d*DP-DP. Solving for d, d = -DP/(MF-DP). Plug in the spell damage formulas for MF and DP to get:
d = (4109 + 1.965*S)/(962 + 0.465*S)
When using 2 orbs, we equate the damage to 1/2 a MF channel, and similar math gives us:
d = -2DP/(MF-2DP)  = (16436 + 3.93*S)/(13289 + 6.36*S)

I know you're looking at those formulas and saying, "Sheytan, no matter what value you plug in for S, neither equation's value changes very much at all!." You are absolutely correct. Spell power is practically inconsequential to this, which means that it will be very easy to tell when we should use fewer than 3 orbs because the threshold to do so is static. This is why the below graph basically looks like two horizontal lines. To the graph!

(Click to enlarge)

So clearly, we don't really want to use DP with 1 orb. We just don't see a 425% increase in damage. The only exception would be when the boss is dying before we'll get to use a second orb. Also pretty clearly, using DP with 2 orbs doesn't take much in the way of procs for it to be a DPS gain! That's awesome news, so go out there and use DP a bit more often without fear.

What non-haste procs give us 124% damage and allow us to use DP with 2 orbs when they proc?
I know you don't want to hear it, but it varies from person to person depending on your gear, so you need to do some testing. You can find out easily by going to a target dummy armed with AffDotsPriest. Put SWP on the target and right click buffs until you have only the one you want to see. Tips to make this process easier: uncheck "hide when not in combat" on affdots so it doesn't disappear after a few seconds, and make a /cancelaura macro to remove all of your procs except for the one you want to see, then spam it after casting SWP! For example, in my gear at 563 ilvl, I found that Jade Spirit is 5%, Synapse Springs is 6%, my 549 Breath of the Hydra is 29%, and just 4 stacks of my 561 BBoY is 31%. It may sound tedious, but it'll only take a few minutes. Knowing combinations can be good, too. For instance, I also know that 3 stacks of my 561 BBoY + Jade Spirit is enough.

Okay, but what about my LMG, Bloodlust, etc.?:
Good question. Haste adds ticks to DP, so we're going to have to figure out how many ticks each combination of things adds. This is pretty easy thanks to Twintop's graph on howtopriest or checking the smart breakpoints of AskMrRobot's stat weight calculator.

Again, DP's damage is: n(1643 + 0.786*S + h(274 + 0.131*S)), where n = # of orbs, S = spell power and h = the number of ticks we get from haste. When comparing the increase in DP across various haste procs using the same number of orbs, simply subtract DP with the increased ticks from DP with 9 ticks, since14,873 haste with no buffs gives us +3 (9) ticks. PI/Troll racial gives +4 (10). Lust/LMG gives +5 (11). PI/Troll+Lust/LMG gives +7 (13). For MF, we just take the haste proc's value and multiply it by either a full MF channel for 1 orb DP comparisons or 1/2 a MF channel for 2 orb comparisons.

I am assuming 14,873 haste from gear for these comparisons, but similar methods can be used if you're not there yet or are stacking even more haste. That said, more Graphs!

(Click to Enlarge)

It's not surprising that using DP with 1 orb is, again, a loss in DPS. It suffers from a double whammy - 1 orb means it doesn't get the orb multiplier, and it's also compared against a full MF channel. Casting DP with 2 orbs, on the other hand, isn't much of a gcd loss, and it gets a multiplier on its initial tick in addition to the extra ticks. I had planned to investigate more haste procs, but a trend is clear, and MF will also stop benefiting from haste procs due to the gcd cap. It's possible that this will allow DP with 1 orb to become more viable in some extreme haste proc stacking situations, but I'll leave that up to you all to figure out if you are so determined.

I fully believe that similar effects are the same if you're at 18,215 haste (you can use DP with 2 orbs for haste procs), since the primary variable is spell power. However, given how resounding these results were, and how until current TC becomes more definitive I'm favoring 14,873 haste, I have not looked into it to make sure. If anything, I think that 18,215 haste will favor 2 orbs slightly more, as it hits a higher breakpoint or two with certain haste procs.

If you have trinkets that give non-haste stat procs, go to a dummy and see which ones give you at least a 24% increase in damage. Chances are good that if you have some recent trinkets, though, they'll do this. Remember those, and when they proc, it is totally fine to use DP with 2 orbs if you won't be able to get 3 before the procs fall off. If you have haste procs, it's also going to be worth using DP with 2 orbs when they proc.

One very common time that you will likely find yourself doing this is at the start of a fight when you didn't get a chance to save orbs from trash or grab some from your fellow Tito owners. You should be able to get 2 orbs before your procs fall off, but they'll all be gone before you get a 3rd. Use those two orbs with all of those procs!

Also, keep in mind that this is based on the assumption that you're constantly doing these things. If something needs to die within seconds and you won't get to use 2 orbs, heck, use 1!

If you found this interesting/helpful, or if you have you always wondered if it's better to do ____ when ____, follow me and leave a comment. I love pondering new situations, so maybe the answers to your questions will end up on here!

Saturday, October 19, 2013

Keep Dots Up During Mind Flay: Insanity!?

You may have noticed that I love Mind Flay: Insanity. The powerful burst with a reasonably short interval not only allows us to blow things up quickly but increases our long-term damage capabilities significantly. However, it can cause problems because it requires us to stand still and cast nothing but MF:I for as long as we can. These days our dots don't last as long (goodbye UVLS+2pc T15). What if our dots are crazy strong because we cast them with mega procs, but those procs fell off already? Should we refresh them early, losing some of that awesome damage, or should we let them keep doing their thing and reapply them later? Let's take a look at how we can maximize our DPS by making the right choices. We can either refresh dots early, and lose a bit of a gcd, or we can let our dots fall off during our blue laser beam turretting action. It seems like a lose-lose situation, but if we do it right it's a solid boost to our DPS.

Do you smell that? A nice, warm chart fresh out of excel!
(Click to Enlarge)

To begin, we need to know how long it takes us to channel insanity, so we know if our dots will even fall off or not. Devouring Plague takes a gcd and lasts 6 seconds. Assuming you're at a decent haste level, this means you have just about 5 seconds left on DP by the time you can start channeling. If we recast insanity right before DP falls off (which you absolutely should be doing!), we can gain another full channel of the dot for great success. Ideally that's 5 seconds, or gcds, of MF:I plus another 2 seconds for the last full channel after DP falls off (give or take slightly depending on how early you refresh it and your haste levels for MF's lengths). We also want to cast Mind Blast on cd, and it's up when we're done with MF:I, so that's another gcd. But if our dots fell off, we'll need to spend another 2 gcds recasting them before they're back on, too. 5+2+1+2 = 10 gcds we spend from the start of DP until we're wanting to deal with dots again. Our dots tick every 2 gcds, which makes the math easy: We have up to 5 dot ticks possible during our DP+MF:I burst phases.

Now that we know our total dot damage possible in the background of our Mind Flay: Insanity phase, we just need to compare the damage gained from the buffs the dots have to the damage lost of them falling off. Firstly, make sure you're using AffDotsPriest, made as always by the amazing Alae. We need to know how strong our dots are, and how much stronger or weaker they'll be if we refresh them, and this tool is too good to not use for this!

If our dots have 2 seconds left when we start our DP+MF:I burst, that means they have 1 tick left, so we either accept that we miss 4 ticks of our dots (5 total possible during this time, and we're only getting 1) or we refresh them early to make sure they never stop ticking. Unless our dots are currently doing 5x the amount they would be if we refreshed, we're going to lose some damage here. This leads us to finding that we need our current relative damage (the number AffDotsPriest shows) to be greater than n/10, where n is the number of seconds left on our dots. If this is the case, then do not refresh them. Otherwise, refresh away.

If you've run into other situations during your spriesting, and you're not quite sure what the best thing to do is, leave a comment. I'll look into it, and we'll see what we can come up with!

When to Refresh Dots for Procs

To launch this blog, I'm going to begin with an essential post that many of you may have already seen elsewhere. It's been recently updated after some changes that came with 5.4, and it just needs to be on a blog dedicated to practical theorycrafting. Without further adieu:

I've been interested in really nailing down an answer to the questions most shadow priests have of when to refresh dots. Here's my attempt to do so, and I'll do my best to explain what I've done. I welcome any discussion about my methods, as getting this information as accurate as possible is the goal! I am planning to expand this soon with more information on how the 2pc extensions affect these decisions, too.

So, let's just start off with the graphs. The reasoning and math behind them will follow. I've graphed when to refresh SWP and VT separately because they are quite a bit different, as you can see.

The Graphs:
(Click on a graph to enlarge it)

You need to know several things when considering when to refresh dots. One is how much time is left on your current dots. If they're about to expire, you refresh them right away, for instance. You also need to have an idea of how many ticks your new dots will last. You only spam SWP (i.e. it lasts 1 tick before it's refreshed again) as a last resort during movement, for instance. You also want to know your current damage increase/decrease compared to your state when the existing buffs were cast. I highly recommend checking out AffDotsSpriest (you'll also need Affdots for it to work). It displays information just like my graphs - if your current procs have increased your damage by 20%, you'll see a "120" on your dot trackers. Thank you very much for writing it, Alae and Ridcully!!

Let's choose an example where you have +30% damage from some int procs, are at +2 ticks from haste on your gear, and the current portion of your fight has very little movement, so you expect SWP and VT to last 8 and 7 ticks respectively. I find myself in this situation very frequently during a raid. Your current dots have 7 seconds left. What do you do? For SWP, the red line for 8 ticks shows that with 7 seconds left it's worth refreshing early when your buffs are at least 100% and have about 8.5 seconds left. You should refresh SWP here. For VT, the blue/gray line labeled 7 ticks shows that you should refresh it early when it has 7 seconds left if you're at around a 10% damage increase. You should refresh VT, too, because 30% > 10%.

One important aspect to look at is 10 second procs. In this case, if you're going to refresh immediately and then again right before the 10s proc fades, you're likely getting around 4 ticks of the dot before you recast it. For these situations, you should be looking at the purple 4 Ticks line. To simplify the LMG proc down to a +30% increase, for LMG procs (alone - no other procs up) you can safely refresh both dots immediately, regardless of their remaining duration.

The Math:
That's how it works. Now I'll explain how I came up with these. Here is where I would be particularly open to peer-reviewed criticism, so feel free!

What I did was figure out the damage you gain by refreshing dots and compare that to the damage you lose by spending that time refreshing dots instead of filling it with mind flay. Recasting one of the dots is half a mind flay channel, so I divided the mind flay damage by 2. Both SWP and VT would be ticking for damage even if you didn't refresh them early, so you need to subtract that out. You will need to spend that GCD refreshing the dot eventually, so refreshing it early is simply a fraction of that GCD. For instance: SWP lasts 18 seconds, so refreshing it with 6 seconds left means you're wasting 6/18ths of the GCD you could have spent casting MF by refreshing SWP early. Thus I also multiply MF by t/18 for SWP and t/15 for VT to get the fraction of MF time you lose by refreshing early. SWP has an initial tick component which is included. For the following formulas, d is your current damage increase from procs compared to what you had when the existing dots were cast. t = time left on the dots, M = MF damage for a full channel, S = the damage from 1 tick of SWP, V = the damage from 1 tick of VT, n = the number of ticks you expect the new dot to last.

For refreshing SWP: d*t*M/36 = dS + dnS - nS

Solved for d: d = 36nS/ (36S + 36nS - tM)

For refreshing VT: d*t*M/30 = dnV - nV

Solved for d: d = -30nV/(tM - 30nV)

This essentially gives us a formula for what we have if our spells are instant cast and we aim to refresh them as close to 0s left as possible. First, we aim to refresh them between the last two ticks, so I adjusted t by -1 for both dots. VT has a cast time, so I adjusted t by an additional -1 for VT because that's the time the dot will actually land, but we want to know when it's a good decision to start casting it or not based on the time it has left now. This seems right to me and led to what seems proper for VT on the graph. If any of you can think of a better way to adjust for these things, please let me know!

Conclusions (tl;dr):
Refreshing dots for procs can be very good. SWP is very forgiving when it comes to refreshing early. SWP's initial damage makes a big difference, and when you're expecting the dot to last its full normal duration, even a medium-sized buff, such as just a lone proc of my Megaera trinket makes it worthwhile to refresh very shortly after it was just cast. VT is not quite as forgiving, but you can still increase your DPS significantly by refreshing it early for procs as well. Pick some numbers you commonly see with AffDotsPriest after your procs, and use the graphs to see how much time your dot should have left on it before you should refresh it for those procs.

Some limitations:
1) The values used for the damges on MF, SWP and VT when making the graph are based on my character's stats, at 560 ilvl. However, the spell power scaling of VT+SWP is nearly equal to that of MF, so this applies to nearly any ilvl for a level 90.
2) I did not factor in the type of buff. SWP clearly scales well with crit due to shadowy apparitions and even more so with 2pc T15. Increased benefit from extra SA procs and dot extensions are not included. I did not factor in haste specifically, either. While I included extra lines for increased numbers of ticks on the dots, I did not factor in the GCD cap of recasting these vs the higher GCD cap on MF.