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Yeppers, good thing to be on the level...
Marv is 100% correct. If a level repeats after 180° reorientation it is correct; even with some apparent displacement of bubble. If not, either base or surface have an interference, some imperfection out of plane to their respective surface.
Process he describes is the procedure, by textbook and in practice. There are conditions that increase accuracy; a master level will displace its bubble just breathing on the vial [temperature change].
A good quality carpenters level can be very reliable too. In their normal use, the check combines level [or perpendicular] along with straightness of a surface; over an extended area of contact. That combination may not always be the best indication of information sought.
Try this experiment. A kitchen table, counter top, etc...demonstrates this as well if not better than a precision surface.
Place a level on a clean 'flat' surface, obviously longer than the level. Observe reading.
Now, level still in same location, place thinnest feeler gauge, shim stock, tissue, number size drill etc, at one very end. Observe new reading.
Next, place level on two identical items at very end; gauge blocks, drill blanks, shim stock. Don't be surprised when a third different reading occurs.
You could with little effort determine practical accuracy by a little math here, working the overall length by the known thickness of spacer.
On many levels, the intermediate pair of marks indicate a slope of 1/4" per foot, critical in generation of dependable drainage.
While base of a precision level can support itself decently in full contact or at its ends, I use that second tendency in leveling machine tools. I suspend the level on good 1-2-3 blocks or reamer blanks. With the spacers separated by a known distance, it is quite simple to better estimate what amount [and direction] of movement is needed to get a machine on two [X & Y] truly horizontal planes.
I used 2 levels, an 8' carpenter for X and 12'' magnetic torpedo for Y on the bandsaw support. To get accurate cuts for long material used in machine frames, this and improved leveling of the saw itself had to occur.
Attachment 23835
As no table connects machine and support there likely is a sag in the level, regarded negligible by reversing level for the same reading. I found imperfections in the concrete floor experimenting with the distance and approximating the Airy Points https://en.wikipedia.org/wiki/Airy_points