79pmooney

This isn't an exact science. Different bikes have different chainstays with different clearance issues. (Dropout spacing, chainstay diameter and derails like the attachment at the bottom bracket and (sometimes) chainstay curvature all make a difference. So does the shape of the crank.) So no one really knows what the minimum BB width is until you actually try everything. And even then, remember this is a fit of a very shallow taper over an equally shallow taper, done by hand. Very small differences in size on either the crank of spindle will affect how far in the crank slides. Now Shimano is pretty consistent with their spindles. But different crank manufactures adhere to spec to varying degrees.

So the end result is that we are all guessing. Everyone here and FSA. The advice of buying a best guess Shimano BB and trying it out is excellent. Then you know what you really want/need.

Aside - I have recently set up two bikes as triples with absolute minimum Q-factors for my knees. Went first to the Shimano BBs I had on hand, measured, calculated and ordered from Phil Wood the perfect bottom brackets. Worked like a charm. My frame clearances are in single widths of cardboard. My knees love it.

Edit: there is also chainline. Some consider the exact, proper chainline to be the word of God but face it, chainlines are a lot more like pant sizes. Perfect is nice but we can wear an inch bigger or smaller. Being off with your chainline will cost you a little an reduced chain life but so will using your rear derailleur and any combination that isn't perfect. Other issues get worse with poor chainlines, but still others get better. Crankset too far in? Small-small is worse for chain stretch and interference with the large chainring but the chain is happier in the low gears and there is less friction in the highest chain tensions. Too far out? Reverse all that I just said.

Ben