Zamir and I spent a bit of time testing the interaction of FS and the 4pc at different haste levels to get a clear picture of what it actually was doing. So far there have been three theories of how it works:
1) at least 6 seconds added to FS always
2) at least 5 seconds added always
3) closest number of ticks to 6 seconds added always
We know for sure (because of the text of the set bonus and comments from Blizzard) that the 4pc will always add a minimum of 2 ticks to FS per extension regardless of haste level, but how the set bonus works exactly (especially at higher haste levels) was important to determine in order to see how it would affect our DPS and gearing choices.
So we both went on the PTR and tested FS extensively at different haste levels. We aimed for specific haste targets where the level of haste should have been enough to gain an extra tick of FS out of the set bonus, and then did testing above and below to see exactly what behavior the extensions created in the DoT. For each level of haste we performed two tests: five single-cast extensions (SINGLE), where we cast FS and then Lava Burst only once; and two LvB-until-expiry tests (MAX), where we cast FS and then Lava Burst on cooldown until the FS DoT ran out completely.
Here's the data from our testing:
20% haste (656 haste = 2.5s ticks)
SINGLE: 1 refresh, 8 ticks each time (6 + 2)
MAX: 3 refreshes, 12 ticks both times (6 + 3*2)
Time for #ticks:
*2 = 5
*3 = 7.5
*4 = 10
29.58% haste (970 = 2.32s ticks)
SINGLE: 1 refresh, 9 ticks each time (6 + 3)
MAX: 5 refreshes, 21 ticks both times (6 + 3*5)
Time for #ticks:
*2 = 4.64
*3 = 6.96
*4 = 9.28
50% haste (1406 haste +WoA = 1.9997s ticks)
SINGLE: 1 refresh, 9 ticks each time (6 + 3)
MAX: 3 refreshes, 15 ticks both times (6 + 3*3)
Time for #ticks:
*2 = 3.9994
*3 = 5.9991
*4 = 7.9988
74.18% haste (1458 haste +WoA +EM = 1.719s ticks)
SINGLE: 1 refresh, 9 ticks each time (6 + 3)
MAX: 3 refreshes, 15 ticks both times (6 + 3*3)
*2 = 3.438
*3 = 5.175
*4 = 6.876
75% haste (1474 haste +WoA +EM = 1.7140s ticks)
SINGLE: 1 refresh, 10 ticks each time (6 + 4*1)
MAX: 4 refreshes, 22 ticks both times (6 + 4*4)
Time for #ticks:
*2 = 3.428
*3 = 5.142
*4 = 6.856
100% haste (899 haste +WoA +EM +BL = 1.4999s ticks)
MAX: 3 refreshes, 18 ticks (6 + 3*4)
Time for #ticks:
*2 = 2.999
*3 = 4.499
*4 = 5.999
149.2% haste (1928 haste +WoA +EM +BL = 1.2035s ticks)
MAX: 2 refreshes, 16 ticks both times (6 + 5*2)
Time for #ticks:
*4= 4.813
*5= 6.017
*6= 7.220
Results from the 20%, 50%, 74% and 100% tests disprove the "at least 6 seconds added always" theory as the duration added is under 6 seconds. Results from the 75% test seem to disprove the "at least 5 seconds added always" theory because the smallest number of ticks for a 5 second addition would be 3, but we saw 4 ticks consistently in all circumstances. As for theory #3, the 74% and 75% results show a transition from 3 ticks to 4 ticks. At 74% haste, (6secs-3ticks) is 0.825 and (4ticks-6secs) is 0.876. The smallest number (and thus closest to 6 seconds) is the former, so 3 ticks are added. At 75% haste, (6secs-3ticks) is 0.858 and (4ticks-6secs) is 0.855. The smallest of those two numbers is the latter and right enough 4 ticks are added. This strongly suggests that the number of ticks closest to 6 seconds is being added, which fits all the other test data too.
In short, our testing confirms that the 4pc t10 setbonus adds to FS the closest number of ticks to 6 seconds always. Zamir has already started working on updating the spreadsheet, so we should have updated DPS values for various gear levels very soon.
Big thanks to Zamir for the majority of the maths (and also legwork) on this project.
