Converted a Motobecane Grand Record but the chainline is way off, no matter which side of the spider I put the chainring on. There's no room on the hub to put spacers on to move the cog outward, so I figured I'd just go buy a shorter BB spindle to pull the chainring closer. My question is, how can I figure out what length to get? Is there a generally standard size that will fit? Or is there a way I can actually calculate it?