I think you did fine explaining.
it's just original potency X Total DMG+ Buffs X DMG+ from resistance reduction. That's general rule for weapon skills and auto attacks.
Then we open a can of worms by discussing whether DoT damage is reduced by piercing resistance (Disembowl) or magic resistance (Foe Requiem)...
DoT tick damage btw is determined at the time of application, so if you apply the DoT with your buffs up, the tick damage remains at the higher value for the subsequent ticks until the DoT wears off.