The maths is pretty simple, but after a while you will know the numbers pretty much of by heart. I don't work out percentages, instead I work out odds. It is then much easier to compare with the pot odds.

For example, you have a nut flush draw on the flop. There are 47 unknown cards. 38 of them do not help you, but 9 of them will give you a winning hand. So your odds are 38 : 9 which simplifies to 4.22 : 1. On the turn the calculation is the same except that there are only 46 unknown cards. For this example your odds are 4.11 : 1. Depending on the pot size these odds tell you whether to call or fold.

Percentages are more useful when you are trying to estimate your winning chances for the whole hand. So continuing the example above you have 9 outs and want to know what percentage of times you will hit on either the turn or river. This is calculated as ....
1 - (odds of missing on flop * odds of missing on river)
1 - (38/47 * 37/46) = 0.349 or ~ 35%

This percentage tells you that on average you will win 35% of all future bets in this hand. So if you have 4 opponents who are all likely to call, then you should raise as you are contributing only 20% to get a 35% return.