I don't know for a mandolin, but for a guitar the percentage of the entire string length for the fretspace for 1 semitone is....
100{[(12th root of 2) - 1] / (12th root of 2)}%
If you want to divide the string length rather than multiply, the reciprocal of this is approx. 17.817, which is called the 18 Rule.
One you figure out the fret spacing for 1 fret, subtract that spacing from the entire string length and repeat the process until you have enough frets.
Re: fret spacing
I don't know for a mandolin, but for a guitar the percentage of the entire string length for the fretspace for 1 semitone is....
100{[(12th root of 2) - 1] / (12th root of 2)}%
If you want to divide the string length rather than multiply, the reciprocal of this is approx. 17.817, which is called the 18 Rule.
One you figure out the fret spacing for 1 fret, subtract that spacing from the entire string length and repeat the process until you have enough frets.
Have fun.