A quick grid to test your bed flatness. You can customize the size to be nearly your bed size and the thickness to be your layer height or maybe 2 or 3x the layer height. Quick print that lets you easily see where your bed is not level.

There is a small square at the origin so after you lift it off the bed you can remember how it was on the bed if you need to. (Now selectable during customization).

The STL file is premade for 150mm bed with 0.2mm layer height.