Please Explain Abilities' Damage When Dual-Wielding

Hello!

I rolled a rogue on Lordaeron and I'm getting to know more about weapon choices, abilities, dual-weilding, etc. I understand that the off-handed weapon has 50% reduced damage when auto-attacking, but how do I calculate my abilities' damage? Take Hemorrhage for example: deals 110% weapon damage (increased to 160% if a dagger is equipped). 1) Is this a one-weapon attack, therefore calculating for either one of the two weapons or 2) is it an attack with both weapons. 3) Does the 50% reduced off-handed weapon damage affect anything in such calculations?

In general it's based on Main-Hand weapon. Abilities that calculate from both are sort of "exceptions", Mutilate, Fan of Knives. Then that -50% offhand comes into play. Tough I don't recall if FoK did 100% of each weapon, meaning it does full damage from your character sheet where -50% is already present. Anyway, that's the deal, it's based on Main-Hand weapon.

Thank you a lot!