Thats what you do when you devalue outs. SSH talks about examples where you have ATs on an 278 2 suited flop. Someone bets and you have to decide to call. You know if the flush comes you have the nuts so you have 9 flush outs.
You know if a T or an A comes you might have the best hand...but you might not so you have some outs. you have to make a guess as to how many outs these 6 cards are worth. You might decide they are worth 3 outs. So you add your 9 full outs and your estimate of 3 outs to get 12 outs and you decide if it is worth a call based on the pot odds.

It sounds like this is is the sort of situation you are talking about.