How much haste is needed to fit two SB between HoG?

[Quantarian 0]

I have been trying to figure this out and have not quite seen a number yet but realize latency etc will also effect this.  My problem is is I can get my haste and mastery about even around 7k each.  Thought about pushing my haste to the extra HoG tick but that would push my mastery to 2k below my haste and not sure if the spread between them would be worth it.  

[Zagam 1,982]

Shouldn't need any Haste at all to get 2 SBs between HoG.  SB's base cast is 2.5 seconds and SF's base duration is 6 seconds.  HoG - SB - SB - HoG would work with 0% haste.