Join Date: Jan 2008
Location: Beaverton, OR
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Have you tried asking the in the Mechanics forum? I don't know a lot about Octalink. Is that one of the standards where the axle length is theoretically paired with the number of chainrings to give you a standard chainline?
In general, given the expected spindle length for a crank, it's easy to calculate the change in chainline that you'd get from a different length bottom bracket. The chainline for a double is the point between the rings, while the chainline for a triple is the location of the middle ring. Generally, you don't want the inner ring to far in, but with a triple, it's good to have the outer ring farther out.
With standard chainlines (43.5/45) and a typical 5 mm spacing between rings, the inner ring is at 41 mm for a double and 40 mm for a triple while the outer ring is at 46 mm for a double and 50 mm for a triple. This is the consensus standard and seems good for most people. Depending on your typical usage, you might be able to get away with some variation.