How often the opponent must fold for a bluff to break even.
The formula is bet ÷ (bet + pot). Betting 5 into 10 is 5 ÷ 15 = 33%. The memory phrase is "25, 33, 40, 50": the numbers for bets of ⅓, ½, ⅔, and 1× the pot. If the opponent folds more often than that, you can bluff.