The old timers method of using a "balance factor" isn't accurate which is why the correct balance results in a different "balance factor" for different engines.
That is why a made a spreadsheet calculator that uses all the different weights (piston, rod, bearings, pins, etc) to calculate both the vertical forces and the rotational forces and shows you a graph of the forces throughout 360 crank degrees so you can see how close or how far away it is from the best possible balance. Then you enter various balance hole sizes till its as good as it will get. see
http://www.dragonfly75.com/motorbike/vibes.html