A 2" pipe end is about the right size for something like a 6-10" mirror and even larger. Use either end, the closed end will provide more surface to grind and the open end will also work but you will have to put a hole to relieve the air pressure in the inside of the cap. You will spend most of your time on the center of the mirror (TOT) and lesser times to the outer parts of the mirror. The tendency is to make a spherical surface anyway so don't worry about it. The radius is controlled by the same method as with a full size tool, by the time you have 50% of the depth, you should be 50% of the diameter carved out. Several of the guys in the workshop have done thier mirrors this way and it goes fast, is easy to do and saves on frustration with tiles wearing out. You do need to do a couple of wets of the coarse grit in order to mate the tool to the mirror and verify that everybody is really a nice sphere but that goes fast.