the valve refacers mostly followed the original contour,just had to keep moving to avoid a flat spot.
Exactly. The valve regrinding machine's fixture does help, in that the rocker is held on a shaft at right angles to the wheel to prevent incorrect motion. What you want is a 2-dimensional wave, not a dome with a curve in both axes. As long as the pressure is light and constant, and the sweep across the radius doesn't stop, a competent result can be achieved.
Experience is the master, but diligence is a good substitute.
