This.

Just create a 50x50x0.2 “cube”. Then let it print 3-4 lines. If the filament is round like spaghetti and not touching each other, you are too high. Lower it. Repeat until the filament is so much pressed down, that it flows to the sides (you can see ridges next to where the nozzle printed). Then you are too low. Ideally, you just have a very flat, connected surface.

As soon as you change the temperature of the extruder or heatbed, you might have to calibrate the z-axis again. Other than that, it doesn’t have to be perfect.