I've never seen a 122.5 used wirh anything but a triple. I always try to use the shortest possible spindle for a single chainring. I like the crank to miss the chainstays by no more than 6mm on both sides.
Using an adjustable cartridge bottom bracket like a Phil Wood or this low buck Miche Primato allows you to dial in your chainline, but you still have to get the length right. I've miscalculated and bought the wrong length before, I've been able to either sell them or use them for another project.
