Diagonal

Largest 3-4-5 that fits (100 mm steps)

Both diagonals should read this when all four corners are at 90°. The 3-4-5 sides run 3 along the shorter side and 4 along the longer one; the third figure is the check across.

Target Diagonal

Diagonal 1 (measured)

Diagonal 2 (measured)

Average of the two

Racking gap (the difference)

Long diagonal comes in by (half the gap)

If the average is well away from the target diagonal, the length or width you entered doesn't match the frame. Check the sides before you push anything.