I just bought a new 60 cm bubble level from a relatively reputable brand - Empire - and I had to go through six of them in the store before I found one that was accurate. I own three longer ones, two of which have been miscalibrated since the day I got them. The third one would have been as well, but luckily I checked it before buying.
It would be so easy to just add a small adjustment screw for calibration. I don’t understand why they don’t do that. Even the more expensive ones I’ve seen don’t have one. I’ve developed serious trust issues because of this.
Also, in case you didn’t know, you can check the accuracy by rotating the level 180 degrees and seeing whether the bubble stays in the same place.
Doesn’t have to be level, just flat. Ox tools example
Our process isn’t anywhere near that elaborate. We’re just making a quick check to see if a level has been knocked out of whack somehow. But we’re not a machine shop either, we use the levels for verifying weight and balance on small aircraft.