What Inner potential should I try to get?

There's a little detail I forgot to mention and just realized might be important. My specific job is Hero

That said, could anyone explain to me how the "x% extra damage when attacking targets with ..." would work with this job considering I got chance attack and there's also boss dmg?

I've had this line many times before (with 10% at Legendary rank) and after a bit of testing, it seems to be a flat multiplier, where x% is added on top of all other multipliers such as ACA, Enrage, and Chance Attack.
EDIT: I believe Rare and Epic rank gives 2-5%, Unique rank gives 6-9%, and Legendary gives 10% only.