Two exceptional fretboard calculators you need to explore if you are designing multi-scale (fanned-fret) instruments. I have been using these websites extensively during design of our aircraft aluminum Exoskeleton™ necks.
I want to thank both website builders for their valuable contributions to the lutherie community:
FretFind2D (www.ekips.org) — Exquisitely accurate placements devised by a math genius with aesthetic skills plus subtle humor. Accurate to at least five decimal places. The data imports into CAD.
Liutaio Mottola (https://liutaiomottola.com/formulae/fret.htm) — Wonderful reference resource, including historical & traditional calculation methods. Multiple fretboard calculators, including extended range.
Email me if you know any other excellent sources, or leave a comment below.