I just do it in my head. If you're drawing to A234 lets say, and you know you need a 6 or 5 to win (say somehow you know opp has 7432A0, you have 8 outs. If this is the first draw you've seen 5 cards, so there are 47 left.

8/47 = 17% chance to hit that 5 or 6 on the first draw. Then lets say you don't hit, on the 2nd draw you have now seen 6 cards, so

8/46 = 17.39% chance

etc etc etc