So I'm replacing the triple crank on my bike with a proper single chainring crank. The old crank is an ISIS and the new one is square taper, so I need to buy a new BB (which is fine 'cause square taper BBs are better anyway).

Problem is that I don't know what spindle length to get for a perfect chainline. I don't even know what ballpark to use and even if I could make a reasonable guess I figure this is my chance to straighten out my chainline without resorting to spacers on the chainring.

Is there a procedure one can use to figure out the correct BB spindle length when you have the crank in hand and you know what the rear chainline measurement is?