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If you're needing something level and you don't know how accurate the spirit level is, its definitely worth reverse checking it first.
Simply place the spirit level on a clean flat surface and note where the bubble is (or the digital reading). Rotate the level 180 and reposition it in the same place. Ideally you'd be on a known level surface and the bubble should be centred (reading 0) both ways, although your chosen surface needn't be level, so long as the indicated angle reads the same (a digital 0.75 would change -0.75)
user69121
lynx
Seriously I thought I was going insane. The level said it was fine, my eyes (and wife) said it was completely wrong: https://youtu.be/rQ9984rfkDI