Although the "rule" as such is close enough, the bigger mistakes are usually made in overcounting your outs, not in figuring out how much each of them are worth.
E.g, K-Qo on a board of 10-9-4, rainbow, two other players in pot. Preflop, you raised in middle position, button 3-bet and the big blind cold called, and you called. After the flop, the blind and yourself checked, button bet, blind called and it's back to you. What's your chance of winning?