Ratios normally, % when in all-in situations. Remember, you should be calculating your odds one street at a time, so knowing that you are 35% on the flop to hit your flush doesn't really help you (that's the % with two cards to come). It's really a simple memorization:
outs-- ratio
1-- 46:1
2-- 23:1
3-- 15:1
4-- (gut shot) 11:1
5-- 8:1
6-- (over cards) 7:1
7-- 6:1
8-- (oesd) 5:1
9-- (flush draw) 4:1
10-- 3.7:1
11-- 3.3:1
12-- 3:1
13-- 2.6:1
14-- 2.4:1
15-- 2.1:1
If you were able to successully memorize your multiplication tables this should be no problem. Really, you could probably get buy with just to bold ones.