"calibrate a level" does not need to be on a "level" surface, and likely not the best to compare to a second one that may or may not also be correct. Set on any surface one way - take a reading. Then swing the tool end for end - should get identical reading. If not, then is not "calibrated" - adjust vial, grind bottom, whatever was the maker's plan - a level should read the same, whether placed North South or South North. Works the same for vertical / plumb - need to get identical reading regardless which end of tool is up - Learn that when building - there is only one "level" / one "plumb" and your tool and his tool need to agree where that is. Like a tape measure - your tape and his tape must agree what is 92 1/2" - many tapes do not agree with each other - only one distance is "correct", though.