Hi Yuan -
I use the same formula - the total pad/ball mating area (after collapse) divided into the weight (in grams) of the package. I have used a not too exceed number of 30 g/ in sq. but have gone up to 44 g/in sq.   So the weight per joint will vary with the number of balls, the size of the balls and the pitch.
Hope this helps.
Regards,
Phil Zarrow

ITM Consulting 
Durham, NH  USA
www.ITM-SMT.com
T: (603) 868-1754
F: (603) 868-3623
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