I was indeed (and I think you’re right, the calipers would need at least to be parallel on their outer edges to work this way).

I’m not sure what rz2000 was doing by (slightly wrongly) rewording basically what I wrote — I get the impression they think I was being full of myself for thinking of a (similar) concept that already exists (despite conceding that it already might) and felt the need to put me back in place.