I'm guessing the answer will be "trial and error", but is there an 'easy' way to narrow down the needed BB spindle length? Like, say, info* on a 49D calls for an asymmetric 118.5mm (double) or 123.5mm (triple) spindle- is one likely to have success with a symmetrical spindle of the same length with spacer(s) to get things lined up properly? Or does one have to add a couple mm to the spindle length when going from asymmetrical to symmetrical or something?

*cross-ref'd from info for a TA Cyclotouriste on Sheldon Brown- not confirmed as to whether this is actually accurate for a 49D....