I line the first element up with a center ruler to the outer flange references.

I can then site everything down the center. It is hard to see from this angle, as I could not get the parallax out of this shot with my little camera. But trust me you can line it up.

Verticle clearances are set a different way in this plan. I still think there might be a way to combine the laser and this technique, just have not thought of it yet.