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.
Well … Where to calibrate it then? You’d need a reference. Just a clear tube with water works all the time. But yes, it has some inpractical issues
You can use the same method you already used to check accuracy in the first place. It doesn’t even need a level surface - just make sure the bubble sits exactly the same distance from the lines when you flip the level the other way around.