I'm already on a 103mm BB. TA track cranks are 46mm chainline.
which model cranks are you using? FWIW if a single ring chainset ends up ~45 or 46mm chainline, this is usually because it is really one designed for derailleur gears. 120mm/5s chainline is ~45mm.
Out of interest, do you check chainline with a 2' steel rule? Just measuring front and rear chainline is pointless as frames are often up to 4mm out. Buying £500 worth of matched Campag track components can still give you lousy chainline, something Sheldon never emphasised enough.
If you want a really good SS setup there is no point in nailing parts onto a frame without checking it first.
1) check the BB shell is square (it sometimes isn't). Lay a straight edge against the chainring (so that the inclination is the same as the top tube) and compare it by eye to see if it is parallel or not. If it isn't, get another frameset.
2) check the track of the frame. You can do this using a length of thread, running from one rear dropout to the head tube, around that, and back to the other rear dropout. Compare the clearance to the seat tube each side. Correct accordingly. If you have a good eye, you can 'sight' the head tube, seat tube and dropouts from the front; no thread required.
3) check the dropouts are at a uniform height, and there isn't a 'tilt' to the rear wheel. To do this fit a good rear wheel, and (with the bike laid down) lay a straight edge on the wheel so that the inclination is the same as the seat tube. Compare and see that the two are parallel. If they are not, double-check that the wheel is parallel to the top tube; if it isn't, this will cause a small tilt error with track dropouts. Tilt errors are difficult to correct, but fortunately are also fairly rare and don't really affect how well a SS setup runs, mainly how well the bike steers.
Needless to say if the frame is even slightly out of track or the wheel is not correctly dished, the rear wheel will likely be slightly skew WRT to the centreline of the bike; this will result in poor running, as if the chainline is wrong, even when it isn't.
You can run whatever chainline you like when all is said and done, but if you want to use proper track equipment, e.g. campag pista hubs, without a lot of aggro, best to stick with a 42mm chainline. No need to spend a fortune on parts to give a 42mm chainline if you don't want to, you can get a 42mm chainline with less expensive parts easily enough.
cheers
