No, I was wrong. They are multiplicative, according this post Total Damage Stacking - #2 by tqFan
So it’s should be something like
DMG * (1 - 0.45) * (1 + 0.8)
I will try asap. One question: is Cindertouch so much better than Scarmarker? Why?
How does this work? I was under impression that all the modifier where unaffected by OS.