Bike Forums - Building up a new frame, reusing components

I'm going to be building a new steamroller soonish, and to save money (in the short term, at least) I'm going to be reusing wheelset/cranks, amongst other parts. I'm trying to figure out what size bb I need to buy to create a straight chainline, and I'm a bit confused. Right now my chainline is about 42mm measured from the rear, and looks straight. The spindle length of my crank is 118mm. Even after checking Sheldon's site, I don't know how to derive the bb size that I need from those two numbers - what am I missing?

[edit] Got it figured now - the spindle length listed for my crank is the recommended bb length.