Running under the hypothesis that the MDF expands and contracts with the humidity and temperature, I'll start each printing day checking the height, simply by running the extruder down to minimum and see if it hits the bed or not. By having a piece of paper underneath, I can tell a good fit from a tight fit. Normal paper is 0.1mm.
Today (nice weather, not rainy, Google calls it 20C and 48% humidity) I had to raise the bed 0.5mm before the paper came loose. So I set the Z offset to 0.4 and ran a test. The print stuck but was a little open in the bottom, and had upturned corners.
Now that I think about it, a Z offset of -0.5mm doesn't make sense when the layer height is 0.35 mm - it would go below the minimum defined by the firmware. Can it do that?
With the slight looseness of the test print in mind, I changed to 0.2mm Z offset and started printing a knitting loom. The first layer looked a bit squeezed, but not squeezed to the point of whiteness. Result: Excellent!
To check if the hobbed bolt is still gripping, I mark with a Sharpie on the filament just above the extruder entry, and if it doesn't move, I press the filament a bit. That has already saved one print.
After the knitting loom came out nice, I decided to go big, and print a dream weaver for my brother's partner. I picked the largest one that will fit my printer, and even so I had to move the corner clip away to make room for the skirt. A giant circle really shows off any and all sideways levelling issues, and my bed is not quite even with the print head. I suppose it would be easier to adjust the height of one of the Z pillars rather than fiddling with the bed supports.
What better time to fiddle with machinery than in the middle of a print? :) I paused, turned the motors off, turned the right-hand Z hold 1/4 turn towards up, then resumed. Not a hitch in the print! But, as is not uncommon when the first layers were squeezed, it later started stripping.
After that, I did a proper left-right calibration by adjusting the Z heights left and right.
Several days and small prints later: Yes, the printer definitely changes height with the weather. Today it's 28C and (supposedly) 50% humitidy (though it feels like more), and the Z offset is 0.6mm to get a good print.
An autoleveler would be good in this case, since the temperature and humidity could actually change within the time of one printing session, and re-leveling between each print would be annoying.
At 16C and 94% humidity, the Z offset is 0.0mm.
I had one print come out with a few layers underextruded before it got started again. I suspect the filament got tangled in itself. What do people do to prevent filament tangles?