I would stake a wager that you're going to need exactly
2482 haste for your 8th immolate tick as a goblin, and
2635 if you are not a goblin. I am noticing a pattern of haste-modified tick speed being rounded to 4 significant digits.
Also, 2635 is the amount of haste at which I was able to achieve an 8th tick through testing.
EDIT: To expand on the notion, let's look back at the observed behavior of 580 haste NOT granting our extra tick. In theory, we need (1.3 / 1.15 / 1.05 / 1.03) = 4.524714% bonus haste to reach that milestone, which maths out to 579.42 haste rating.
Let's look at what the tick speed is at this haste level - that's the amount we need for the bonus tick.
(3 / 1.3) =
2.3076923.
At 580 haste rating, nongoblins would theoretically have enough to clinch that extra tick. However, it's getting rounded up.
(3 / 1.3000562) = 2.3075925, or
2.308. We are just too slow to get that extra tick at this point.
Let's check it again at 581.
(3 / 1.3001533) = 2.3074202, which now rounds down to
2.307.
EDIT EDIT: If you're a goblin, your 7th immolate tick should be observed at 448 haste rating. I will slap together a spreadsheet calculation once I finish with work.
EDIT EDIT EDIT: Rough draft of the
spreadsheet is done! Yell at me in a PM if something is inaccurate.