When I packed the laser, I remember thinking that I would only back out one screw, so that the other two would retain the alignment. But that was 3 years ago now, and I don't remember which one it was. And I vaguely remember that I had to back out another a bit when I was actually pulling the tube. These days I'm addicted to Microsoft OneNote, so I would have written that down!
Step one was to check that the aiming laser still worked, as a quick check that the mirrors were still where I left them. (See earlier in the thread for the red pointer laser that drops into the beam path when the door is open). I moved the carriage all around and verified that it still worked:
I actually found my original alignment helper jigs, but I don't have the matching laser pointer anymore. So I printed a new set. These are rings with an OD that matches the laser. One has a pinhole (not as an aperture, but to help aim), and one holds a laser pointer. By aiming the laser pointer in the rear holder at the pinhole, it should force the beam to be parallel to the final position of the actual tube:
Here you see the guide laser dropped into the beam path being hit from behind by the jig aiming laser. The red arm with the guide laser was propped out of the way for the actual aiming (it also raises automatically when the door is closed).
It's actually easier to verify the aiming of the laser pointer by looking at the back side of the front disc. It's probably even easier if you use a dimmer laser:
Here's where it was hitting initially (the carriage is at the front right corner to get the longest beam path):
And here's what it looks like with a little adjustment (also tested over the whole area of movement):