Bottom bracket spindle length?

How does one determine what bottom bracket spindle length to get? I'm asking what I need to measure on the bike to make sure I get a length that will give me a good chainline.This is for a newer road racing frame that had no bottom bracket when I got it so I have nothing to compare it to. I'm using a square taper crankset (FSA carbon) and have found FSA ultimax BBs online at reasonable prices. I would prefer having the big chainring line up with middle of the cogset. Is there a measurement or combination of measurements I can make to come to a reasonable number?