According to Sheldon Brown, a Phil Woods bottom bracket with,
This crankset,68 mm Cartridges will have approximately 2.5 mm of the rings extending from both sides of a 68 mm shell. The 68 mm cartridge in a 73 mm shell will allow the rings to be flush with the shell.
asks for a 103mm bottom bracket for a 42mm chainline (assuming a 68mm shell).
My question is, since I have a 73mm bottom bracket shell width do I need to account for this in the spindle length? Should I therefore get a 108mm spindle to compensate for the 5mm lost on the shell width? Thanks.