You're doing the math right, but your conclusion is wrong. If the bet to pot percentage is 10% and your percentage of hitting is 13%, that means you WANT to call, because you'll hit 13% of the time and lose 10% the other 87% of the time.

If your chance of hitting is greater than the money you put in, you should call. Ratios are just a reciprocal of percentages, so the logic gets reversed.

chance of hitting % > bet to pot % -------------------- CALL
pot to bet ratio > not hitting to hitting ratio ------- CALL

Hope that helps....