the general way i figure percents/odds is with some sule i saw on some site that works

after the flop you multiply your outs x 4, and after the turn you multiple your outs by 2 and that is the % to catch one of your cards

example: flush draw on the flop, 9 outs x 4 = roughly 36% to make my flush

that is close enough to the actual percentage for me and its easy to do in your head on the spot