A follow-up with a couple more pictures showing the use of the laser:
Step 1: get it all nice and level. Note that the laser line has to pass through the centre of the rear axle as well as the centre of the BB shell. That means we're really on the centre plane of the bike.
So now step 2, without moving the laser, drop the seat tube down, so that the laser can see the HT. It's also sweet that with it rotated around the laser is still hitting the centre-line of the axle (although this isn't actually critical at this point-- what matters is just that the ST goes right back to exactly where it was).
