The equity you need to call.
The formula is call ÷ (pot + bet + call). It depends only on the size of the bet. Example. A bet of 5 into 10 (½ pot) needs 5 ÷ 20 = 25%. The memory phrase is "20, 25, 30, 33, 40": the numbers for bets of ⅓, ½, ¾, 1×, and 2× the pot. In Hi-Lo games, compare your share, not your chance of winning.