I printed out your widget. I used the 3mf file. Success the first time! 
The bed is within .25mm, which is perfectly good for me as a hobbyist. One spot back where it does the nozzle cleaning routine it went .3mm.
Great project and answered if i was close to flat or not.
p.s. I did have to file down the underside of the pointer so it did not rub on the dial plate. Maybe my magnet was too thin. Didnt measure it.
Cheers for wonderful tool!