A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Morris
Date: 2019 Jan 13, 14:18 -0800
Bob, to calibrate a level is not a simple task, but it is reasonable to assume that a good make will have a vial ground to a segment of a circle rather than some other shape, the the bubble should move uniformly.
If you simply want to level some surface, find a fairly level flat surface. On this surface will be some line that is level. Place the level on it and rotate the level until the bubble is central. Place a heavy straight edge alongside it to keep the alignment and reverse it end for end. If the bubble remains central then the level is good for use. If it does not, adjust the vial to remove half the difference and repeat the exercise.