Different field, but in my work I find calipers are about the only measurement tool I can actually trust. The starting point on most tapes and rules is iffy enough, but when you get to single-purpose tools similar to chain checkers they can be all over the map. I once paid $80 for a very simple tool (two moving parts and a spring) and it was designed wrong so that it was doomed to be exactly 20% off. . . and had been for the entire 30 years it had been on the market, so I scraped off the scale and calibrated it myself. If you really want a 100% accurate chain wear gauge [Does it really matter? I have no idea], sit down with a file and make one. :-)