Figuring sucks.
My mirror should be a sphere. It isn’t a sphere. I’ve introduced a “turned down edge”, where the edge is ground lower than the rest of the mirror and you have to remove all the glass in the center to fix it. You can see it in these ronchi test pictures, each taken after a few sessions of 30 minutes of total polishing. The straight lines show that part of the mirror is spherical. There’s an annulus shaped around each one where it isn’t straight lines. I hope these pics show that it’s getting better over time…