What is the formula used to determine bat weight based on the weight of the individual and how accurate is it? It seems like I figured it before and it came up heavy compared to charts I've seen based on the girls height. If I'm looking at a chart and a girl is 4 feet 8 inches tall I believe it says to use a 30 to 31 inch bat (I would say 29 to 30). Now, for a 10 year old most people recommend an 11 or 12 drop which would put the weight at 18 to 19 ounces. Based on the formula what should a girl weighing 90 lbs at 4' 8" use?