JONG BB question

I've taken on a project building my first bike from scratch. All seemed to be going well until I came to a issue with my bottom bracket/crankset compatibility. My crankset documentation says it needs a spindle length of 113/118mm. So my question is, which do I get, or do I split the difference with a 115mm. I know it has to do with getting the chainline right, and a friend told me it might depend on my seat tube diameter (which is 1 1/4" (31.8mm)). What size bottom bracket do I need?

what bike?

Bottom braket width is dependant on the frame. What frame do you have?

What cranks do you have? This will determine the type of bb.

Square taper? Octalink? Isis? x-type? Mega-exo?

I'm building off of a nashbar touring frame. I know that I need a square taper, 68mm bottom bracket. It is just a matter of the spindle length.

and the cranks are sugino xd300s.