Most accurate mirror spotting technique????

I have question regarding the most accurate way to center spot a large mirror. The recommended way to find the center is to measure from the "edge" in many places, or to make a template with a compass the same size as the mirror. But most mirrors are really slightly larger than their advertised size to compensate for the beveled or champhered edge the mirror has so the mirror will not easily chip. This edge is not typically uniform and is variable around the circumference. So what edge is best to use, the actual outside edge of the mirror, or the outer edge of the figured top surface. I am not a mirror grinder, but it seems to me......
If the mirror was figured first, then aftewards the edge was ground off, seems the better edge would be the outer mirror edge. But if the edge was ground off first, then the mirror was figured, seems the better measurement would be the final edge of top mirror.
Am I thinking correctly, or an I all wet...
When is the mirror edge ground off, before or after figuring??
Thanks in advance....

The method I use is to place the mirror on top of a large sheet of paper (face up of course) and trace the outline circle of the entire mirror on the paper. I then carefully cut the circle shape I traced out. I then make two folds. The first fold is in half. Then I fold the half again so it is now quartered. While still quartered I take some scissors and snip the tinest piece off the center. When I unfold the paper I have a perfectly centered hole square shaped. I then place it carefully on my mirror and stick my center mark tape in the cutout hole.

Lawrence Regira

I like Jim Fly's template made with clear acetate. It has circles that are in inches for the diameter of the mirror and a place for the triangle center spot to be attached with cellophane tape by means of small holes with just enough stick to hold it until it is attached to the mirror. Then it easily pops loose. The template is reusable and comes in sizes for up to 30" mirror I believe.
I used to use the trace and fold method, but this is much better.