Hi there,
I've got two suggestions:
1) Buy your mirror, don't grind it! Unless you are determined to have a hand in every inch of the scope and money is no object and you don't care how long it's going to take, my advice is to buy. (and yes I've tried making a mirror before...)
2) Join the ATM list here:
http://www.atmlist.net/mailman/listinfo.cgi/atm Where some very knowledgeable people hang out and would know the answer to your question better than most. The BIG names of the ATM world are there and you'll get the very best advice.
Don't forget there is the atm and DIY forum on this site too and questions like this might be better asked down there - you don't seem quite the beginner to me and some of the people on here that might be able to help would see you question down in that section sooner than they would see it up here.