I have a set of the same gauges. The concern is that the contact surfaces are flat, rather than conforming to the inside curve of the tube. It appears to me that the surfaces need to be modified for better accuracy. That's why wall thickness tools sometimes use pointed pin type pieces for the inside contact surface of the tool.

Regards
Ken

I prefer wood to plastic, leather to nylon, waxed cotton to Gore-Tex, and split bamboo to graphite.