After you find the center zone, I set my dial indicator dial so that the needle points to 0. This merely makes it easy to see the other zones as I just read off the value the needle is pointing to.
Micrometers don't have this ability so you just record all of the values and plug them into the data reduction program. I use TEX and it doesn't care what the numbers are referenced to (you can, if you wish, use the absolute value of the distance to the mirror!).
The fun you are going to be having is getting the grayness of the two zone holes equal and the eye is better at seeing which one blinks out first than the absolute value of the brightness. Also, at the outer zones of short FL mirrors, you can run into the zone blinking from one side to the other and this is normal.