This test is designed to show any deviation in distance between the nozzle and print bed throughout each adjustable corner to ensure quality, level prints. Save yourself some filament and use this test routinely to re-level your print bed.
For best results, stop print after the first layer and check homogeneity. A level bed will produce consistent texture, even if the nozzle is too close/far. Raise corners that appear "patchy" a quarter-turn at a time, then repeat test until satisfied. I find it better to raise the lower corners than lower the high corners, assuming you've already modded the z-endstop with a fine tuner.
Designed for a 220x220 mm build plate, but the object itself is 200x200x20 mm (perimeter) to leave clearance for the leveling screws.
This test helped me determine that my A8's aluminum heat bed was warped from the factory, so I just cut a piece of glass to clamp over top. I perform this test weekly and as needed. Additional file formats available upon request.