Burst dmg uses the same formula as normal attacks but has an extra multiplier tacked on, so it looks something like:
Burst dmg = normal attack dmg * (a + b)
a = hime/weapon specific - most LB3 SR = 3.5x and most LB3 SSR = 5x
b = burst multiplier buff such as PF, exceed weapons and what not. It is capped at 500%
Total burst dmg per hime is soft-capped at 1m but can be increased to 2m by exceed weapons and certain hime skills.
Burst streak dmg = total dmg from all soul/himes * 1/(7-n)
Where n = number of participants in burst (if n = 1, there is no streak dmg though.) So, basically at FB, streak dmg = 50% of total dmg. This goes up to 70%-ish if you have elemental advantage though. Burst streak also applies to all targets, not just the target of the burst, so if you're MVP racing in a rag, there's a strong advantage to bursting before all the adds die.
Ability dmg does not factor in at all for burst dmg calc.
tl;dr - for MLB SSR, FB is about 7.5x normal attack dmg, higher with stuff like PF and exceed.