I use this model to see Z calibration defaults.

I have a square bed with 4 screws (one on each corner)
I print this model and then I check the way it's printed on the first layer.

• If you can see filament lines, it means that the nozzle isn't close enough to the bed, so the screw of this corner need to be loosen (IMG_20181206_114415.jpg).

• If the nozzle makes noise or get clogged during the first layer, it means that the screw of this corner needs to be tighten

• if the first layer is perfectly smooth, the calibration is good (IMG_20181206_114357.jpg)

on my ANET A8 printer, I added an Anycubic glass so the first layer grips well.