After four hours of using the ring tool with 60 grit, I sufficiently reached the goal-spheroid of FL 78″ that we decided to move from ring tool to tile tool.
(1) we traced the circumference of the mirror over a mat of porcelain tiles and cut the mat into the circle with a wet saw.
(2) we covered the surface of the mirror in foil, laid the tile mat face down on the mirror surface, and built a dam with flashing and duct tape.
(3) we mixed the dental plaster, poured it onto the mirror, and let it dry for about an hour. (The dental plaster being exothermic, the “cake mix” began radiating noticeable heat … not enough to fry an egg, but remarkable as one held one’s hand over it.)
(4) we removed the tile tool from the mold and ground the topside (non-tiled side) by sliding it (topside down) on the sidewalk outside the community center. The result is a grinding tool of the same diameter as the mirror: the tiled side has the same curve as the mirror, and the topside is more or less flat.
Now to 80 grit …