Having an incorrect flange focal depth with prime and zoom lenses will obviously make all of your witness marks useless. A zoom will not hold focus over its entire length. But it seems that if you're focusing by eye that you can still get sharp focus when the cameras FFD is off. What I want to know is if this is CRITICAL focus, can you obtain a sharper image, or is there more resolving power to the lens when the FFD is spot on? (Assuming the lenses are all calibrated and have been checked.)