Or, with the digital variety, you close them, set them to Zero, then open them to fit over the item to be measured and close them again, and extract the measurement from the digital readout.
YOU introduced vernier calipers to the discussion, and said that the use of this tool would require some manner of shim to measure around the lip of the brake rotor being measured. A vernier caliper would most certainly require this sort of adjustment to the proper use because they are not well suited for this application. There are calipers with curved jaws that can reach around the lip on a brake rotor, and a micrometer is an even better tool to use. (actually, I use a micrometer because I have one, but it might not be the superior tool to use, it's just the one I have. It's hands down better than a vernier caliper though.)
When a suitable mic can be found for $20 (USD), there is very little justification for not having one. When the make or break measurement of of the brake rotor is a matter of thousandths of an inch, shimming the wrong tool with a twist drill, then subracting the shim amount from the measurement just seems silly to me.
Well, if one possesses a mic and doesn't know how to use it, I can't help much. That guy should take the rotors off and take them to the shop to see if they are still serviceable.