JS lightweight pistons for stock rods in 850 balance factor change

If JS lightweight pistons are installed in a stock 850 with stock rods, what is the resultant balance factor?

I just checked the site again and found the "missing link" with the answer: it will increase the stock balance factor of 52% to approx 58%. I'm actually considering installing them in a 920 that I believe to have the RGM 920 kit with "slightly lighter than stock" pistons, so I'm thinking maybe 59%?