| Knowing the percentage of time you are going to make your hand in one or two cards is just part of the equation.
I use the 2%x2 method for complex situations where I have several types of "outs" to calculate at once, IE an open-ended straight draw AND pairing my high card (8+3=11). With the more common situations like flush draws, I have the exact number memorized.
Either way, knowing how often your hand will hit is just half of the problem...that's where you are going to need to convert the percentage into a ratio (IE, 5-1) so that you can contrast that with the bet/pot ratio ($10 bet into a $50 pot, etc.).
There are times when something like a gutshot four-outer justifies a call because the pot is so big in comparison to the amount it takes to stay in the hand. This type of situation is common when you have a hand with numerous callers and/or raises.
You may only have an 8% chance of winning the hand....but if the bet is $10 and there is already $150 in the pot, you have the right odds to call.
Similarly, you could have a hand like a straight flush draw that has a much better chance of hitting....but there is so little money to be won in comparison to the bet that you should not call. This can happen in a very tight game with few caller/raises....or in a situation where someone goes all in in NL.
There could be $100 in the pot, but if someone raises $300 you are getting pretty cruddy odds for a drawing hand - not even 2:1. At that point, you can only call if you think you are ahead.  | 
