I have an attachment for my old Sioux valve grinding machine. Here are the instructions:

https://picasaweb.google.com/crenwe...Gv1sRgCIXI3pjjpLucaw#5698773529606842578

Two cones center the rocker where the shaft goes. There are different radius settings you can chose from. This is done by trial and error. You then move the rocker tip on the selected radius on a freshly dressed stone until the indented places are gone. Its nothing real scientific, but I've been doing it for years.