I thought about ID referencing the pin to a specific hole in the lathe nose to replicate the setup with the chuck mounted. I know my collet chuck & 3-jaw have an orientation that is about 1-thou closer to concentric. In this case its for a 4-jaw chuck so no real concern that way. But apparently the 3-jaw has same recess dimensions if I decide to get it one day.
For reference the back plate recess is 95mm (3.740") diameter & 4mm (0.158") depth from catalog specs, although I measured ~0.200" actual depth on the chuck.