damage operations

After reading the Treatise on Combat Mathematics by Ensign, I was left wondering how +%dmg stacks, before the calculation of weapon masteries. Is it a function of multiplication or addition?

Here's a hypothetical...John Doe finds a max 15>50 axe. If he decides to customize it, his damage output will increase in either of the following ways:

(6-28 dmg) * 1.15 * 1.20 = (8.28-38.64 dmg) [23.46 average]
or
(6-28 dmg) * (1 + 0.15 + 0.20) = (8.1-37.8 dmg) [22.95 average]

Granted, the difference is only a little over two percent, so it's not a huge deal...but does anyone have data to confirm one way or the other?

I did a little bit of testing and i believe that the second option is whats true. I don't have anything to prove that but im pretty sure.

I too believe it's the second one.