On my old Sioux machine, two cones align the rocker. Therefor the rocker face will be parallel to the shaft, and I think that is what is important. The attachment has has about a half a dozen radius to chose from. I do this by barely touching the stone while I rotate the rocker. When it contacts the surface the same in front as in back I consider it the correct radius. I have never seen a machine that uses a more scientific approach to the radius, but I'm sure they are out there. But I think my method is close enough for a stock GM engine that is 50 to 75 years old. Here are the instructions for the attachment. Possibly this will explain better how I do it.
https://picasaweb.google.com/crenwe...Gv1sRgCIXI3pjjpLucaw#5698773529606842578
