How does damage % per hit work?

Just to clarify something from what some people said

The following is a somewhat similar situation but it's additive and not multiplicative correct?

"Ancient warding gives 30% damage and spirit surge gives 15% damage. That would be 45% damage and not 49.5% damage correct? And if we add say a 10% nebulite (just for example), it would be 55% damage and not 64.45% right?"

Ancient Warding + Spirit Surge is additive to each other. They multiply your damage. From what I recall, any total damage is multiplicative against this, but I'd have to double check my own Mercedes later.

To slightly hijack this thread, are the passive boosts to Muspelheim, Edge Carnival, and Savage Blow from Boomerang Step multiplicative or additive? Also, same question for the 150% extra damage from Dark Sight vs. +150% bonus damage from Killing Points for Assassinate? And even though it's not damage, the buffs to Pickpocket, Meso Guard, and Meso Explosion from Pickpocket?

The only damage bonuses that are additive are the airborne ones. Does the Boomerang Step bonus require the monster to be airborne?