cheap levels can be calibrated by simply making fresh marks on the glass. Absolutely no need to smash them...
cheers
Don't know how to recalibrate fixed bubble spirit levels. Of the two that I had access to; one was a plastic no brand that had deformed and the edges were no longer straight. Whilst the second is a rolson brand level and the three levels aren't adjustable.
it isn't obvious to see how to do it but it is not difficult once you know how.
1) find or make a truly horizontal (or, for the other bubble, vertical) surface.
2) make fresh marks (using marker pen, tape, w.h.y.) on the exposed part of the vial.
The first part is slightly awkward to do, but once you find a flat surface (or make one eg by using paper shims under one end) the bubble sits in the exact same place with the spirit level either way round. If you make temporary marks on the vial, then turn the level round 180 degrees, you can see if the surface was truly level or not. To make the best marks, use indian ink on the vial, and cover that with transparent sticky tape. Or cut a length of opaque tape the exact same length as the bubble, and put that alongside the bubble. Maybe there are other ways of making marks that might suit you better yet.
Repeat this process with each bubble and each reference surface on the body of the thing that you wish to use. Obviously if the reference surface on the level isn't itself flat, the level will only give a true reading if placed against a larger (and perfectly flat) surface.
You will find that even supposedly good quality levels are actually calibrated poorly such that you could easily make a small error.
With practice you can easily recalibrate the most accurate spirit levels this way. For example if you have a decent engineer's level such as one of the (very excellent) Rabone Chesterman models
these have graduations on the vial that correspond with tiny angles (far smaller than you might get with a digital level BTW). IIRC the marks (for a 12" level) correspond with elevations of just 0.004" in 12", or 0.0019 degrees, and you can read fractions of a mark if you want to. You can adjust the vial on this type of level using the locknuts at one end; IIRC increments of about three degrees on the locknuts gives vial elevations which correspond roughly to one marked line on the vial.
Obviously you are not going to be able to calibrate your cheapo level to this kind of accuracy but on fixed vial levels you can nearly always improve it; even by spacing the lines apart by the same distance as the exact length of the bubble helps accuracy. Once you have you reference surface (or calibrated level) it is easy enough to check other levels against it.
Spare vials (to replace broken ones) can be bought cheaply enough eg
https://www.ebay.co.uk/sch/i.html?_nkw=spirit+level+bubble(starting at about a quid each) and need not be super-accurately mounted if you are prepared to calibrate them afterwards.
With any spirit level, you can improve accuracy (where space permits) by simply laying the (short) level onto a longer length of something straight, like a length of good quality aluminium extrusion. (BTW if necessary you can check the straightness of the extrusion or the 'composite level' by using the same technique as above).
hth
cheers