I need to find a bottom bracket for the build I am doing. Never bought this part separately before so I am having some trouble choosing the right length. I'm thinking of going for nashbar on this one (paired with nashbar double crankset). Width is straightforward enough, I get that, but is there any way to calculate what length would work best? I have it narrowed down to either 108 or 113mm. I'm leaning towards 108 because I figure I could always add spacers if the chainline is too wacky.

Thanks for any advice.