Making something that needs accuracy? Test your printer first.

Print this test. You are looking for 150mm on each side and equal diagonal dimensions.
Possible issues, Measure near the top of the print so you are not dealing with elephants foot.

If the outer print is not 150mm - <0.6mm loose (or tight) belts can be an issue. If it is >0.6mm off steps per mm should be calibrated.

If the diagonals are not equal - This means your axes are not square to each other. You will need to physically adjust your printer. There is a marlin tweak to adjust skew but you would be best to get it physically as close as possible first.

Print instructions

Make sure no size compensations are turned on.