As mentioned, unless you have a special brake disc caliper, you need to use some sort of spacer to clear the lip which forms on the edge of the disc. I use a couple of small 1/2" round by 1/4" thick magnets. Micrometers work fine but are much more accurate than you need. Even previous generations measured to the nearest 0.001" (new ones 0.0001"). You only need to measure to the nearest 0.025" or 0.1mm.
If your brakes are operating smoothly, the values won't change much at different locations. And depending on the backing plate it may be hard to measure several places unless you remove the discs. Not recommended.
