I've been cutting glass plates and mirrors with my CNC milling machine machine for years. In this video, I describe a few tips and the general technique that I use.
Clamping the glass plate to the table is the critical part of the process, and so I built a jig that allows the glass to be held laterally with shims, but does not require a high clamping force, which would crack the glass.
.085" dia diamond burr
1-3 inches per minute feed
Cut depth .130" (full material thickness)
Flood coolant with soluble oil cutting fluid
Older post: http://benkrasnow.blogspot.com/2008/08/cnc-milling-glass-plates-and-mirrors.html