Measure the width of the fretboard, call this c
Measure the height of the fretboard (could be tricky without verniers), call this s (sagitta)
The radius can then be found using this formula:
[color=#000000][font=Verdana, Arial, Helvetica, sans-serif]r = (4s[/font][/color][sup]2[/sup][color=#000000][font=Verdana, Arial, Helvetica, sans-serif] + c[/font][/color][sup]2[/sup][color=#000000][font=Verdana, Arial, Helvetica, sans-serif])/(8s)[/font][/color]
[color=#000000][font=Verdana, Arial, Helvetica, sans-serif]Haven't tried it myself, it's all derived from Pythagoras. Might try it tomorrow on a known radius.[/font][/color]
[color=#000000][font=Verdana, Arial, Helvetica, sans-serif]Can't claim credit for the maths. I just enjoyed a pleasant mathematical half hour researching it.[/font][/color]
[color=#000000][font=Verdana, Arial, Helvetica, sans-serif]Dario[/font][/color]