Does anyone else here use slips of paper on the bolt face as a makeshift headspace gauge?

I do this at gunshows and don't buy, unless the seller allows me to test for obvious headspace problems.

I've done that a few times, not so much at shows, but to rule out or confirm any suspicions I might have if I"m having problems with a rifle. It's not perfect, but I don't keep a lot of headspace gauges around, so it make a good rough-and-ready gauge.

How can this measure "Headspace" without a correct cartridge case?.