I went through this exercise.
I decided that the best way to do it was with the secondary removed.
With my wife's dressmaking tape measure, I carefully measured (several times!) and marked the exact spot on the opposite side of tube, having first determined the exact circumference of the tube, and the distance from the front equal to the center of the focuser.
Then I installed the focuser, adjusting it until my laser hit the marked spot.
Then install the secondary and collimate, knowing that the focuser is square to start with.