Go read my mirrormaking article at www.access1.net/bobmay and the casting, etc. will probably become easier for you. I'd probably stop at about .020" short as the mating of the tool to the mirror and the fine grinding will get you there. The mirror making group that I work with has used the technique very successfully a large number of times to make mirrors. One of the guys is just starting to polish a 4.5" F15 for solar work and he is probably one of the few that didn't need to hog out (finished sag on a 3" dia circle is .0082) the glass first.