This is the stage I am at. I am using 220 grit with a centered stroke with total amplitude of about half the mirror diameter. This is reducing the center flat on the tool (which you described) and the corresponding ledge on the outer edge of the mirror. It is important to complete this in order to remove the ledge from the edge of the mirror. If there is no corresponding ledge in your mirror and only a flat center on the tool, I don't know if it is important to grind it out. Seems like it is. I would.
I'm keeping the mirror on top, but if your curve is already deep enough, you could alternate having the mirror on top and then on bottom.
Mitch