I wasn't sure so I looked it up and found what I assume is the same post you stumbled upon. It appears as though you're correct, it only works with multiples of 5%. That being said, I just tested the damage multiplier and it works fine. So a dmg mult of 2.1 will deal 210 not 200 damage. There is one small issue though, for some odd reason the red crit floating text "210!" will display the wrong number in those cases. It's still dealing the correct damage the text is just wrong.