After having a lot of issues with bed leveling on my Prusa i3 MK3, I decided to build these squares to see exactly where the issue might be.
It was designed in Fusion 360, and I included the file should you want to adapt it to a different printer.
If you want to exclude any of the lines just set their thickness to less than your printer's capability and your slicer should exclude it.
Remember to print without a skirt or brim! If you want to include a skirt, scale your dimensions accordingly.
It takes about 7 minutes to print.
The parameters with the defaults in brackets are:
LayerHeight => Self Explanatory (0,2mm)
SquareDimension => Dimension of the squares (20mm)
Bed_X => Build plate X dimension (250mm)
Bed_Y => Build plate Y dimension (210mm)
LineThickness => Thickness of the lines between the squares (0,48mm)
MidLineThickness => Thickness diagonal lines between the corners and midpoint. (0,48mm)