You've been going too slow because your tool is almost worn out. The secret to grinding fast is to mark the 3 largest holes and when they go away (you can't find them anymore) it's time to go to the next grit. You will always see several holes somewhere on the mirror that are larger than the others. That's a fact of the beastie. The question is whether those holes are the same ones as before. Do 2 wets and check. The other thing when getting to the smaller grits is to check for the reflection of light glancing off of the mirror. Start at about 2 deg. or so of an angle off of the mirror and gradually increase the angle until the light turns orange. At this point, move the light over the mirror and verify that the center and edges are about the same level of orangeness. At 220 grit you can start doing this test. A properly ground mirror with a short wet will show an even finish. You spent a lot of time on the 80 grit because you had to make a hole - now you can go a lot faster and 5-8 good wets are often enough to go to the next grit size. BTW, your curve is almost to the finished value. You shoult be about 3% or so long to end up with a desired value without having to put the mirror on the bottom. Almost all of the grinding is done so don't panic on the disappearing tiles. Next time you grind a mirror, use a iron pipe end cap to do the carving out of the hole and THEN cast your tool with the tiles embedded in the plaster. You'll enjoy the second mirror more than the first.