I finally got an XLKP collimator, and first things first - I screwed up and ordered 0.5" too short. But how can I tell if I am "close enough" to the focal plane? At some point right before the focuser reaches the end of the travel, the "hall of mirrors" multiple reflections in the offset pupil view goes away. Is that good enough? (P & #2 reflections looks about the same size anyway) I know that if I get the focal plane obviously wrong, then #2 offsets relative to P in the offset pupil. Since the AC axis is defined by the top lip of the focuser, I can just lathe a ring to raise the AC up a little more if needed.
On to the main question, which is "why use the carefully decollimated primary procedure"? It has me look thru the center hole to line up P and #3, but #3 is really dim and fuzzy, and its movement seems relatively insensitive to the secondary collimation knobs. I would much rather jump to the last step with the well-collimated primary and just use P and #2 in the offset pupil to adjust the secondary. At least #2 is bright and sharp, and I can carefully align based on slivers of gaps between the nuclear symbols. And then alternate back and forth between this and the Cheshire.