I drew this up as a quick test to see how level my CB build plate is.
The idea is based on other designs available throughout thingiverse for other printers. This one is more sized for the CB's build area.
Print it at 0.2mm layer height and it will be one layer thick. Adjust the build plate screws as needed while printing to get the best results (do level your build plate before printing), if the infill lines do not meet up, move the plate up, if they are thin and see-through, move it down.
If you have seen other versions of this, you will have noticed that they are nearly all made up of just rectangular shapes and so if you are wondering why the circle in the middle - it is because fully defining lots of rectangles was boring.