Setting the Z height on the first layer, as most of us know, is critical for adhesion and a goot print. I have created STL files with different thickness to quickly check the Z height. The Part is one layer thick and 15mm X 15mm so it prints quickly.
My printer has a BLTouch so it is easy to change the Z offset to achieve the perfect first layer. I arrange this part in the center of the bed in my slicer program to generate the GCODE file.
When I want to check the bed level I add more parts and arrange them in a grid on the bed surface, I use a 3 X 3 grid.