I recently wanted to do a larger cutout (~19.5" x 29") but could not get it square. I have checked the x and y lengths and they are equal. As a test I used the LCD and drilled 4 1/8" holes at the extreme corners and measured the diagonals. I was off about about 1/4". I then unfastened the legs on the x axis (my longer axis) moved the frame a little bit and tested again; same result. I tried several more times and it would not change. I finally moved the x axis by almost 1/2" noticeably moving the frame and ran the test. Once again the same distance off on the diagonals. I am confused as to what might be causing this.
With the origin on the lower left it is the diagonal from lower left to upper right that is always the long distance. I have been moving the far x rail to the left.