It’s this part that is the mystery to me. If no one has a better solution, my idea was to just lay an un-mounted spindle of known length gently across the open BB shell approximately correctly oriented to where it would be if completely installed, then slide the cranks on and run a straight line to the center of the cassette. The problem is, if the straight line is wide or short, it will be just a guess-estimate by how much. Surely there is a better way.