I have levelled the bed on this printer more times than I have printed anything useful, and over the holidays I finally worked out why it never stays put. It was never really level in the first place. It was level at the four corners, where the springs and the thumbwheels are, and then it sagged or bowed everywhere in between. A piece of paper at each corner tells you the corners agree with each other. It tells you nothing about the middle.
The thing that fixed it was not a better levelling ritual. It was a glass bed and a fresh set of stiffer springs. The stock springs compress over a few hours of heat cycling, so a bed you levelled cold at the start of the day has quietly drifted by the third print. Stiffer springs hold the setting. Glass gives you a flat reference that the stamped aluminium plate simply does not.
Then I gave up on paper and did it with a feeler gauge and the nozzle hot, because the nozzle is a different size hot than cold and a print happens hot. 0.1mm, same reading at all five points, corners and centre. First layer went down like it had always known how.
The eternal war isn't levelling. It's pretending a measurement at four points describes a surface. It doesn't, and the printer has been patiently telling me so for months.