What bottom bracket spindle length do I need?
 

I'm in the process of trying to convert my late 80s Miyata Ninety into a single speed. I'm having a hard time figuring out what bb spindle length would be best to achieve the optimal chainline for the bike. Any help?

It seems like Sheldon Brown would just visually sight the chainline and change rear hub spacers or BB axle if necessary. You may need to just experiment.
http://www.sheldonbrown.com/fixed-co...html#chainline

See if there is a bike rescue nearby with part bins, or maybe your LBS will let you try a few out to find the right length. You can then swap for or purchase the proper BB axle.

Do you have the original spindle? If so might as well use it as a guide. It's hard to say what specific length you need...since your back hub will affect the chainline...