Already checked the assembled frame is square and plumb (so not racked/skewed) using a framing square, or something similar that’s large and square?
https://azab2c.com/make?s=squar&v=2_0&r=1.4
During assembly, discovered my build wasn’t as square as I thought, had to redo some work. Despite efforts to square, I still had different sized gaps on the left and right side. Ended doing some test Crown prints and using Klipper skew correction to get Square prints (in XY), even if I couldn’t get the frame physically ‘True square’ in XY.
Edit: Just noticed V1E MP3DP v4 doc has section on squaring.