Bumping this up instead of starting a new thread.

What's the best way to measure the stem clamp diameter? I have a digital caliper set to use, but the question I can't figure out is will it be accurate since the stem clamp moves in/out..

I have a bar in each major diameter but I don't really love this method, it scuffs the parts. I'm also not sure if I can tell if a 25.4 bar in a 26.0 stem is actually the wrong fit.