I don't really have a dog in the race esp. as relates to pool tables.
However, dealing with precision levels has 2 basic components.
1.) what condition is the casting & base in; and is it adequate to the task?
With used levels, this may or may not be a factor for pool table techs to understand or address?
I think i posted links to enough information to confuse most people, but provide those interested with several standard configurations and also competing sets of opinions on which makes sense for what app, to form a choice and consider whether it matters for their own situation.
Side note: some machinist users/practitioners stone the base of levels and gages "every time" between uses to look for that dent burr or scratch furrow that could corrupt a reading. I think that is poor practice, but it is sometimes necessary. It is also a reason that some well used gages including levels can be found to deviate from the manufactured condition. Whatever Pico's opinion, warped bases are not unknown.
All of this or none of this may apply to setting up pool tables.
2.) if the bases are proven and as preferred for the field of use, checking vial calibration is, indeed, simple. Not quite "trivial" due to factors a few of us have posted, but certainly quickly accessible to anyone.
Providing information seems to irritate and confuse many.
Others can sort through it and make choices to improve both confidence and efficiency.
smt