Hi precision Z lash test

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 T