i tend to use the following as an estimate - it's pretty close.

take the number of outs you have (you say you're ok with that) and for the turn multiply your outs by 4. for the river, multiply your outs by 2.

so for example, if you have two over cards with no other draws after the flop then you have 6 outs to hit. 6 x 4 = 24 so 24 is your percentage of hitting an out which is near enough 3 to 1. so you're looking to have better than 3 to 1 odds in terms of the pot. In this case if you had to bet $2 for a chance of winning a $10 pot it would be mathematically correct to do so.

this is extremely simplified and doesn't get into implied odds and so on (how many more and potentially bigger bets might you have to call depending on your position? remember this is the turn i'm talking about in that example, do you expect to win the hand here or will the river come into play? what are your reads on the table - could you be drawing dead even if you hit your card? etc)

however in terms of trying to get the concept i found this useful. hope it helps.