If we could make the top beam itself move in and out based on material thickness it would alleviate this problem without needing to re calibrate. In fact, your setup is already most of the way there. If you flipped the Unistrut that the beam mounts to, it would give you that level of adjustment.
you are talking about two different things.
mrfugu is talking about the distance from one motor to another
you are talking about the distance from the beam holding the motors to the back
of the frame.
changing the front-back location of the beam doesn’t require any calibration.
changing the distance between the motors, even a tiny bit, requires calibration.