2 Ailment types per skill
Several ailment stacks per tick
each ailment can stack 30 times (if skilled)
I gain +5% overall damage for every single stack of every single ailment type (up to 5*5%).
This means basedamage*2,75. when I have 7 elements stacked to 5. Both for skills and for ailment damage.
I tested this with 4 different ailments right now, and I got roughly the right number (+/- 1 damage per tick on the character sheet.)
I specifically used an Ailment of which I have no +damage of in the passive tree. (i.E. "+15% Fire damage" passive nodes etc.). In this case, lightning damage, just to be sure - not that I think it would make a difference because it's probably adding the 5% after all the calculations from equipment and passives.
If It would buff my poison damage on skills and ONLY my poison ailment damage, I wouldn't do 5% more damage. The buff to ailment damage would be less than the buff to skilldamage.
My skill would gain +175% damage, my ailments would only gain +25% damage each.
That means overall dot damage would only increase by 25%, while overall skill damage would increase by 175%.
The way it works right now:
Skills gain +175% damage - each, Ailments gain +175% damage - each. = 100% damage increased by +175%.
Working as intented in my book.