I could not get the bed 100% on my printer to be level. So I created a simple cross check plan to print. This is a one layer at 2mm high and when printed, you can see if there are any high or low spots on the bed. You may also have to rotate X axis by 90 degrees so it sits flat on the bed and Z axis at 0.
The picture shows it on a 200x200mm board, but can be resized to fit any board. Originally designed for a 140x140 board.
Hope it helps.