I moved the insert to the crosshair cup, encoder position 2217749, and stepped the encoder up 30000 steps (~1mm, to 2247749). MCC called reporting lumi ion chamber trip most likely because we hit the top edge of the cup. I stepped the motor down in 30000 step increments 9 times (~9mm in total, exact encoder positions below) until we hit the opposite edge of the cup, then moved the insert back to what we approximated as the center (encoder position 2067749). From this I calculated the encoder offset to be -116320, the crosshair wasn't seen in the scan so this is only a rough estimate.
Encoder positions: 2217749 - starting position 2247749 - lumi ion chamber trip 2187749 2157749 2127749 2097749 2067749 2037749 2007749 1977749 1947749 - lumi ion chamber trip 2067749 - final position approximated as center of cup.