Originally Posted by
Spoonrobot
...I use a dental caliper (or outside diameter caliper) to measure a few spots around the rotor and retire once the lowest spot falls below my preference.
I use an Iwanson gauge, they are also good for measuring rim brake track thicknesses. Like this:
https://www.ebay.com/itm/Crown-Gauge...cAAOSwoetdo-kn
There are also little ball adapters which fit onto a micrometer to measure past a ridge:
https://www.ebay.com/itm/STARRETT-MI...wAAOSw8V9d0D8C