If this is a hypothetical question, then what you want to do is the rough grinding with an iron tool. Use an iron pipe end cap about 1/4 or so of the diameter of your mirror and get a rough curve (about 80% of the sagitta that you want) into it with the iron tool. Then is the time to cast your plaster tool. Just lay the tiles down on your mirror, make a nice high dam with some duct tape and pour which ever the stone material plaster/dentalstone/hydrocal that you want to use. Grind some more with the tool after it's set up good and stay with the #80 grit until the tiles are almost completly curved. Then is the time to start going down through the grits and finish the mirror. If you've already worn out a set of tiles at the edge, just cast another tool if you want. You will have to go through the problem of fitting the tool to the mirror again so keep a good track of the ROC of the mirror and adjust the radius with whichever is on top. I personally have never worn a tile tool out this way and one of the guys in the group is doing an 8" binocs this way and he got all the way done without wearing out his tiles. The secret is to not do the initial grinding with the fast wearing (as fast as the glass) but rather something that doesn't care.