Originally Posted by
TallRider
I use a fair bit of trial and error.
But if I need to buy a new bottom bracket, and the original intended spindle length for the crank (for proper chainline when used as a triple) is known, it seems like this basic arithmetic should be useful.
You are going to take the granny ring off, right?
Your guess looks good. On a Shimano triple I measured, the chain ring spacing is closer to 6mm with calipers.
You'll be within 1/2 mm.