I'm putting together a commuter for myself that needs a new BB. Where I'm buying them there are lots of options in terms of spindle length.

The BB is for a Surly cross check, which has a 68mm wide, standard English threaded 1.37˝ x 24t . First off, what's the 1.37 x 24t mean? Is that the thread designation (another way of saying it's an English thread?)

The BB I'm looking at comes in various sizes. Obviously if you wanted to put a triple on the bike you'd need a slightly longer BB. I'm putting a 1x setup on this bike with a shimano Nexus 8 speed. The BB comes in widths of 107mm all of the way to 122.5mm, what's the correct way of choosing this? Obviously certain people want/like a certain Q factor, so they could choose for that reason, but what's the "actual" way of choosing one of these? I'd imagine I'd want to choose the right length to make a nice straight chain line to the cog on the nexus, but... uh... how do I come up with that info?

Thanks! I've never had to buy a BB myself before. My new bikes haven't needed a new ones yet and my old bikes are all cups and cones.