The design of the final driveline calls for a 4 degree downangle for the propshaft. This is closer to "horizontal" than would be if the stern tube was mounted flush against hull surface as it came from the factory (it would be about 8-10 degrees is my guess). It is desireable to have the shaft closer to horizontal as possible, getting the most forward thrust. But, this is a Westsail and it has a canoe stern, which have a tendency to "squat" under power.
Using laser tools, it was fairly easy setting up the marks to align and center the shaft. The goal is to mark a target on the inside support where the propshaft will intersect, at the 4 degree downangle. It must be lined up and "shoot" through the shaft opening. The opening was cut at the factory and is a little offset to starboard. I dont know if that was intentional or not. It could be to allow shaft removal easier (with rudder installed). Modern fin keel performance boats have propshafts significantly off center for this very purpose. Anyway, the slight offset shouldnt affect handling when under power.