I have a Zee crankset on a 83mm bottom bracket (downhill bike). The Shimano manual says to use two 2.5mm spacers on the drive side, and one 2.5mm spacer on the non-drive side. However, this presumes the rear hub has a 150mm spacing, typical for downhill bikes. My bike has a 142x10 hub, when I use the spacers as specified, the chainline is completely off, I can't even get the chain on the largest cog. Since I'm not using a 150mm hub I obviously need to change the spacers, but I have no idea how to figure out how many spacers of what size I need on each side?
83mm bottom brackets were designed specifically for use with a 150mm rear dropout width. You would be able to shim the bottom bracket across 5mm to bring it to where it would sit on a 73mm bb to align the chain line but you would be pushing your non drive out 10mm which would have an anatomical impact on the rider. This is a significant amount of q-factor imbalance and could quick become an uncomfortable bike to ride.
The simplest solution is to swap the crankset for a 73mm version.
The best solution I found so far are chainring spacers: http://sheldonbrown.com/chainline.html