How does damage % per hit work?

Oh and the damage from Killing Points for Assassinate too?
I bet all the shadowers who have ever existed before me have had a lot of success testing something that's been in-game for 3 months and hasn't hit GMS, which is where everyone here who has a Shadower besides Tay plays.

implying it won't be multiplicative.

It's fairly obvious it is due to patterns and consistency. Go nuts and test it yourself.