The rule I follow for young snakes is
prey size equal to snake girth, feed for slow growth. As long as the snake is shedding regularly and growing then it is getting enough food. This method requires the owner to keep detailed notes tracking progress. What you should never do, is over feed in an attempt to get faster growth. In fact in almost all cases slower growth is better for the snake. A snake that is growing slowly, on just enough food, will live longer than one growing faster, being power fed. I actually think the 10% or 15% percent rule is way too much food for anything bigger than neonates. First BP's are really sedentary snakes. Second, they a be prone to fatty livers. Third, in the wild they have to search for food so they do not get fed every time they are hungry. Feeding a snake whenever it is hungry leads to this. http://ball-pythons.net/forums/showt...ht=fatty+liver
In my opinion, the most important part of feeding is observing the effect feeding has on the animal. Ball pythons should be round, but not overly fat compared with the head. Watch for a concave look or visible bones, these indicate too little food. Keep accurate track of weight. This is the best way to make sure your python is growing slowly. Note: that right before sheds snakes can look malnourished/dehydrated, so if bellies are pink, colors are dull and eyes are blue, this is completely normal. Do not increase food. In fact it is best to stop feeding till after shed.
As your snake reaches sexual maturity it needs much less food. Try to keep weight relatively stable with good body tone. Note: Breeding females may need different care. That is advanced husbandry that should be explored when needed
David