I shifted my mirror up the tube of my Parks 8" to get focus to my pentax film slr years ago and all I did was mount the camera . Then I unbolted my mirror cell andcarefully slid it up the tube a bit at a time until I could get focus in the camera. This gave a good idea of the distance needed to shift the mirror without too much measuring. Then carefully drilled holes at the new position (just measure same distance up from each original hole) and remounted the cell. Worked fine. Cheers Richard
