It depends on if you put a taper under the riser. If .002, then reverse a .002 on the form so the riser still fits.
Anymore, I just make the form ,then fit the riser to it and figure out the radius with the lams under it and mark on the form with 29" R or whatever it is. Then cut the riser on a swing jig on bandsaw, sand on disc sander and go.