Bed level test, super accurate!

Update 27.1.2023: Added edge strip with rounded corners and pre-sliced print file.

Use filament color that differs from your build plate color, 0.4 mm nozzle and 0.2 mm layer height!

The gaps between lines are 0.4 mm wide. If the gaps between adjacent lines come narrower, or there are no gap at all, your bed is too close to nozzle. If the filament is not adhering to bed, or lines look very high, your bed is too far from nozzle.

You can also clearly see your bed plate flatness, if there is any bumps etc.

In picture below, you can see how uneven the bed plate is. More orange color means nozzle is far from bed, and when you see more build plate color the nozzle is closer to bed.