Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3071733.

Attempt to measure flatness of bed using dial indicaor in extruder mount.

Disclaimer; this may collide with the frame and or handle of your printer!

Update 29082018 tested and works. Auto nozzle height was off by 1.5mm when compared to extruder, slightly more than I was aiming for to preload the spring; but if an issue it can be shimed.

Unfortunately even the modest spring in the indicator is more than the pressure the bed can take without moving so the results will be skewed.

TL;DR - Dont buy an Up Mini.