Quick question for Referees: do you use AC (Action Complexity, or DC) with ability checks, and if so, how do you...

Quick question for Referees: do you use AC (Action Complexity, or DC) with ability checks, and if so, how do you decide the difficulty/complexity rating? Do you just wing it or do you have a method?

(Also, if you use only penaltes to the roll for difficult task, how do you decide on the penalty amount?)

Thanks!