As I recall, Howie Mitchell's book How to Build a Dulcimer and Play It (After a Fashion) (published by Folk Legacy; available with or without the recording) has something on this. I think that he worked it out to imagine the fretboard as the hypotenuse of a 1:17 right triangle. By measuring the short leg's length onto the hypotenuse/fretboard, you get another 1:17 triangle. Continue until you run out of fretboard. Considering that the book was written before pocket calculators were commonplace, htis is some neat trick.