At its core, burst damage is {normal_attack_damage * (A + burst_buff)} + B
A and B depend on rarity (of the main weapon or the kamihime herself) as well as whether there's the improvement from 3* or not. A's the important one, B's just a small constant.
For your soul, A varies from 2 (R) to 4.5 (3* SSR). B varies from 1500 to 2500.
For kamihime, A varies from 3 (SR) to 5 (3* SSR) or 5.5 (awakened SSR). B scales at the same rate as for the soul. The table on the wiki actually doesn't list what A is for R's. Not that you'd use R's if you can avoid it... but I can say that burst damage overall is pretty lackluster with my fire team consisting of 3 R's :P
And to be complete, burst streak damage is based off of burst damage done that turn. Sum it up, then multiply by {1 / (7 - N)}, where N is the number of people. So you really want full bursts to make that 50%. Then multiply again by elemental advantage or disadvantage if there is one. If it seems like it can be double dipping for advantage here, it is. And then multiply yet again for any burst streak buffs, but those come from eidolon passives that you wouldn't be using if you have other options.
So in practice, if your soul leads off the burst streak with elemental advantage, expect the burst streak itself to add ~72.5% damage on top of all of the individual bursts.