That is FretFind backwards! I've seen this question in a number of places, "I have a X scale length but I want to add a fret or two before the nut, how do I calculate that distance?" This page should answer that very question. The page is a simplified version of the classic FretFind. The trick: negative tones per octave.
Remember to use like units for all the measurements!

Distance from bridge to known fret:
Enter number of frets to calculate:
Tones per Octave:
Round inches to nearest :