I want to make a jig for this so I layed it out in Cad to try and see what's really happening.
I started with a 16" radius at the nut, and 20" radius at the 20th fret. The fretboard width has already been tapered. I'm keeping the fretboard center at .250" for the full length. When I lay in the radii at the nut and 20th fret, I found that the side of the fingerboard will be about .010" shorter at the 20th fret than it is at the nut. Is this what is to be expected?
If I change the nut radius so that the side of the fingerboard is constant for the full length, I end up with a nut radius of 11.6". Seems a little tight. Or, I can go the other way, keep the 16" at the nut and the 20th fret radius becomes 27.6". I'm probably over thinking this and making it more complicated than it is.
Should I just go with the 16" at the nut and 20" and don't worry about the slight taper of the side of the fingerboard.
(I just like this guy)