Bed Leveling Calibration Test

This is a test object for fine-tuning bed leveling and the Z endstop.

The basic object is a rectangular spiral with line width and line spacing both equal to the nozzle size (in this case 0.4 mm). It makes it easy to spot if the nozzle is at correct height: ideally, line width and line spacing should be equal when printed. If lines are wider than spacing, then the nozzle is too low. If lines are thinner than spacing, or if the object doesn't stick to the print platform, then nozzle is most likely too high.
It's quick to print and comes off the bed easily, since it's a continuous piece of plastic.

The whole test pattern consists of 9 such objects: 8 placed along print area edges and 1 in the center. This way you can easily see which way the print platform is tilted along the X and Y axis.