How to select bottom bracket spindle width? I'm trying to rebuild a bike that I got given free of charge. The bike had no bottom bracket in when it was given to me. How do can I select the optimal spindle width please? I know it's 68mm shell.

Edit: I don't have a local bike shop nearby so I can't take it to one as per suggestions in a previous thread. 

Edit: http://www.chainreactioncycles.com/shimano-un55-square-taper-bottom-bracket/rp-prod71369 is the bottom bracket that I need. English thread, 68mm x Y. Unfortunately I'm unsure of what Y should be. By the look of Sheldon Brown I need 47.5mm from the middle chain ring from the centre of the seat post (measuring to that sort of accuracy could be fun). This is where I'm baffled. I could order a sacrificial bottom bracket to measure what size I need by seeing how far out of 47.5mm I am. Is that what I'm going to have to do?

Edit: I do have a crack. It was a spare one I had left over from a few years ago. It's a triple chainset one by SRAM called Truvativ Isoflow.
You will have an easier time selecting the correct spindle once you have already selected a bottom bracket, because the spindle size is typically specific to each bottom bracket. Once you have the bottom bracket, you should be able to nail down the spindle size either from the manufacturer's instructions or on Sheldon Brown's database:

http://www.sheldonbrown.com/bbsize.html