Center spotting primary

What is your favorite method of center spotting your primary? what medium should I use to spot it? (ie Paint, marker...) Give me some ideas. Thanks

Look at the picture of the cooling fan system I just posted and you will see the mark on my mirror.
It is a hole reinforcing circle for paper made to reinforce paper for binder use. It is white and self adhesive and easy easy to install and see at night. It can be removed with alcohol if needed.
Floyd

I use a router table to make an 1/8" plexiglass circle exactly the same diameter as the mirror face. I make a 1/4" hole in the center of the rough cut disk to turn it true. I slide the shank end of a 1/4" bit - perhaps with a little tape around it to make a snug fit - so about 1/8" protrudes. Then I place a fiber or hard rubber washer on the protruding shank and dab a little bit of rubber cement on it. The disk is then carefully centered on the mirror, and a little pressure flexes it and pushes to washer against the mirror. While holding it, I pull out the drill. If you need a bit more clearance in the center for your mirror put a few bottons of double sided foam tape around the edges (leave the paper on the side toward the mirror!).

The plexiglass also makes a nice cover for storing or shipping the mirror.

Clear skies, Alan