Great advice here already but I'll give you my down and dirty way to do this with any square taper. I assemble the drive side and take it to my favorite bike shop where they have the 4 popular sizes of Shimano. I hand press it on each bb to see which looks like it wouldn't interfere when tightened. It usually gets me plenty close and the Shimano bbs work well. This doesn't help when you have esoteric stuff but it will get you a working setup for cheap.