If you are testing the level by sitting the ‘bubble’ on the glass and you’re sure the base’s spikes aren’t digging in to the floor, check the ball bearing seats.
I find it’s quite easy to insert these wrong way up - which loses 2/3mm of height.
One way to test any difference in the seats/balls and glass would, of course, be to put them on a level surface (e.g. kitchen work-surface) and then test. As Richard mentions, the bigger the ‘bubble’ the better as, IME, the smaller ones are less accurate.
…and it’s always worth checking the accuracy of the ‘bubble’ - just swap it around lengthways.
Anecdote - I bought 2 small ‘bubbles’ from a well-known auction site to test my TT platter and when placed on a known 99% flat surface, the bubbles went in different directions!
I use a combination of the Measure app on the iPhone which has a level function in it and a compact Stabila level as I’ve found theirs are somewhat more accurate than your stock DIY store variety.
And to add something else into the mix I use an electronic inclinometer (make the noise … heheh) as that is just about the best I have used and I don’t have to squint to see the bubble !
Put it on the list of jobs to do whilst stuck at home!
Something with a display with a good level of accuracy is the way to do it though ribbing aside.
I was trying to check how accurate the iPhone app was as that seems to do a pretty good job and gives you a visual feedback as the screen goes Green when it’s level. I then use the Stabila one as a sanity check.
