That contact patch is supposed to be convex. If yours is flat, or concave, it's worn. That's the whole reason for having the tooling on the older Sioux valve grinders, so a rocker arm tip could be re-radiused during a valve job. When casehardened stamped rockers got popular, the rocker refacing tooling was not included with the valve machine because the hardened rockers were not supposed to be dressed at all.
Jerry
