A quick course on setting/ calibrating adjustable levels. Or non-adjustable, for that matter. Tape off a perpendicular area (right angle) of intended reference surface. Ideally the work should be draft free, clean surface, and stable temperature between plate and level. Set the level 'in' right angle boundary of tape and observe reading. Slide the level around and position as identical to before as possible. I've taped down all kinds of objects to be 'tooling stops'. If the level is accurate the reading will be the same; [emphasized] even off a whole bubble, long as identical both directions. Then slip two equal strips of clean feeler gauge, gauge pins, small dowels, needle bearing rollers, anything with a ground surface, under the level. The reading still should remain, after eliminating errors in contact of level and reference surface.
Or a user could send to a cal-lab for a pricey tab; betting they will not take the same pains to insure reading, and certainly not at standard temperature of your shop.
But the cert will look good in a frame...
Another trick for levels is a tapered leaf. Any angle will work, face mill (not grind) a decent strip of good steel (mine is A2) of around the width of your favorite level. I suppose mine is 1 or 2 degrees something, taper starts out .020 thick to just over 1/4" in 7 inches.
Put a small mark at convenient distance from end on your level, like an arrow point of masking tape, 10" and 12" work well on this side of the pond. So now you are setting up new mill, shaper, lathe, grinder, jig bore, broach, rotary head milling machine, plasma table, surface plate; Did someone send you a check$$!?
The first reading will be off. Slip the taper under the low end and move in until level is zeroed. Mark the taper with a pencil (why you don't grind it), remove and check with a caliper how thick that area is. Lets say it's .019 in 10". That equals more or less .0019 per inch. The distance between feet of the machine touching the floor is 91". One side is low .1729 inches, and very close to how much is needed to raise that end nearer to being level. You could even calculate thread pitch of the jackscrews and turn them close to the needed amount. What if the taper is too thick? Dang TM51 told me to make it too thick!
No it isn't; put a feeler under the 'high side' a bit more than its minimum thickness of taper, you can read down to a couple thousandths. Oh, that TM51!
When you are crawling on hands and knees leveling a 10-pad boring machine or lathe, you'll be glad you made that taper.
Of course it is usable to check any sort of opening or gap as long as it fits.