snipcod

I am doing my first fixie build and have run into something I just can not quite figure out myself. I have searched this forum with great luck so far, but now am coming up dry. Any ways I have a Motobecane Mirage that I am building. The bottom bracket was toast so I have been looking at ways to replace it. I have found the Phil Wood makes cups that should fit mine which I belive to be Swiss threaded. In a effort to save a bit of money I was thinking about a Shimano UN-72 bottom bracket cartridge. Found them for about 40 bucks vs Phil Woods that are 120. Where I am lost is what size do I get and how do you figure out the chain line? From what I have read it is effected by the cartridge, crankset and wheels? I plan on keeping my old wheels for now so how do I figure the rest out. Not sure on what crankset I want ether something under 70 would be nice. Thanks for any help

Retro Grouch 

Here's what I do:

1. Install your selected crankset on the bike using any bottom bracket that you happen to have on hand. Also install your rear wheel.
2. Lay straight edge against your chainring and see where it lines up vs. your rear cog.
3. Now you know how far inward or outward you need to move your chainring. Acquire a BB that has the right spindle length to do that.