thank u all guys.
Mattia, i know that jig, i read it somewhere (used for the radius of the fingerboard), but it is a bit complicated for me to set that thing up (in large scale). but i'll try. maybe it will work better on the back, because the curve would be simply circular.
hope in a good precision.
P90, thank for the link, i tried to do a little research, but i found almost nothing.