I can hear all the chorus of groans.... "Oh no not another one grrrr!"
Well hang on I believe this one is different in a subtle but very important way.

I had a non linearity problem with my Z screw system which was tricky to properly identify. My full height Z accuracy was set great up to 300mm, but yet at lower Z heights everything was apparently reduced or compressed by somewhere around 0.6mm. This also caused excessive back pressure with extruder bumping on the early solid layers (after The first layer). This was caused by the early layers not increasing in Z adequately, so no space for the nozzle to freely extrude into.

It turns out that I had a Z mechanism backlash issue. It really only showed up when Z action reversed direction before laying the first few layers at less than 1mm thickness. Resulting in compressed lower layers. So I made this simple precision test pyramid to enable checking and calibration of small Z dimensions right down to single first layer thickness. I just print it then measure the steps thickness with vernier calipers or a micrometer.

So The steps on this pyramid are precisely......