This is exactly what I was talking about in my first post in this thread:
Pistol: If you're like me, thinking about 3:1 is much more difficult than thinking about 25%. I think part of this is because so little time is devoted to probability in school systems (perhaps rightly so); however; we always worked with percentages.
So, here's a chart that can really make things easier. In no-limit, you're constantly running across similar bet sizes. If you just memorize a few of these, you can quickly estimate how often you need to be good and then apply the 4/2 rule to your outs to see if you can call profitably or not.
2x pot - you must be good > 40%
Pot - you must be good > 33%
2/3 pot - you must be good > 28%
1/2 pot - you must be good > 25%
1/3 pot - you must be good > 20%
1/4 pot - you must be good > 16%
As you can see, in nlhe, most of the time you'll be between 20% and 33%. That makes it rather easy.
So, using the 4/2 rule, you can see how easy it is to apply this.
You have 9 outs with the nut flush draw, and your opponent goes all-in on the flop. His bet was a pot-size bet.
The 4/2 rule: you multiply your outs times 4 since you'll be seeing both the turn and river. This gives you 36%.
Your opponent bet pot, so you know from the chart above you need to have at least 33% equity. You have 36%. Call.
Not bad, huh? And you're not messing with any ratios in there.