I mounted a dial gauge to the Z axis of my CNC to see whether the cutting bed face is plane. Related blog post: https://blog.schallbert.de/why-vacuum-table/

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I spent the whole day adjusting the vacuum table by putting thin sheets of metal where the table was too low and applying more torque to the fasteners where it would come up too high. Cannot get it 100% level, though I find it good enough for my purposes.