I'd make it $12 to go. Just a feel thing - that's my usual raise for this kind of situation unless I have a read on the player (or the hand he's holding) that says he'll call more. He could easily have a straight, a mini-flush, or something dumb like two pair here - or as you say a badly played set - and you want to make sure he feels obligated to call. If he has a flush he'll probably put all the money in anyway.