NL expert throwing star handling's damage boost

Does anyone know if this skill's damage boost to avenger is additive or multiplicitive? because my highest hit non-crit at gigantic spirit vikings with lvl 1 handling is about 32k with a range of 6259~9629.
9629 * 357% * 90% (PDrate) = 30,937
9629 * 350% * 107% * 90% = 32454, which fits

Could somebody perform a check on their NL and/or check that I didn't miss something?

Few threads down on this forum:

Just want to take a look at how effective this is.

- 25% boost to Avenger, Drain, etc. - confirmed ingame to be a 1.25x multiplier, which is a really nice damage boost.

Is that what you were looking for?