The spindle length is determined primarily by the crankset, not the frame. As the old BB was asymetrical (assuming that you are comparing with the right one), to determine the correct symetrical BB just take the distance end to end and add to that the difference between the distances from each race to the end (example - if the total is 100, race to long end is 30 and race to short end is 20, subtract 20 from 30 and add that 10 to the overall length for a symetrical BB). Also, if this is a nutted spindle, ignore the threaded parts sticking out when measuring. The left crank arm will be further out but the chain line should not change.