I measured the focal length of the mirror at around 29". The time before that I measured the focal length I projected the moon onto a piece of wax paper, and then measured the distance. That wouldn't work this time, as we didn't have a moon in the evening sky. So instead I put a red LED light some distance (d1) away from the mirror surface, focused it on a piece of wax paper, measured the distance of the wax paper from mirror surface (d2) and used the formula 1/d1+1/d2=1/f, where f is the focal length. I did this twice and averaged.
The mirror did have a chip on the back side, and I was worried that this may have set up stresses and distorted things, so I tested it as described in this post
, and it was fine. Hurrah.