I'm dealing with some BB spindle width issues myself, and here's the consensus I've seen:

Figure out which rear wheel and cog you want and order them. Figure out which crankset you want and order it. Keep the current BB in for now. Install the wheel/cog and put the crankset on your current BB. Your chainline will likely be off, but you can use everything installed to measure how far off you are.

Say you have a 45mm chainline at the wheel, and 50mm at the chainring. You can then measure the width of your current BB, then buy a new BB with a spindle that is 5mm narrower, giving you a pretty straight chainline.

That, and don't worry too much if it's a mm here or there, it's not quite THAT sensitive.