I can see know that I should have provided at least a little more detail. Yes, for my purposes we are talking about tapered spindles. It seems like someone would have come up with a formula for this by now. I have an old steel frame spread to 130mm rear that I want to build with 9 speed cassette and a triple crank.

My only thought is to slip a known length spindle through the BB shell and slip on the cranks, then run a straight line from the center of the cassette to the center ring, try and measure how off the centerline is, and subtract or add. This doesn’t seem as professional a method as I’d hoped for however.