Hey Inoc,
to move the machine to the four points, did you use G1 commands entered into command entry field of the MDI tab?
Have you measured your four A̅B̅, B̅C̅, C̅D̅, and D̅A̅ to be exactly 800 mm? Especially to check if A̅B̅=C̅D̅ and B̅C̅=D̅A̅? Otherwise you would not have a rectangle (or a square in this case with Woodworker)
Did you check your machine for coplanarity? If your front left corner is 1 mm higher than the right rear corner, this would lead to the diagonal to be .07 mm longer – the distance you mention (113.2-113.13)
