i believe ball pythons are supposed to eat prey that is around 10% of their weight, so your snake should be eating a rat that is 35 grams or so. i always feed rats to BP's, mice just don't quite cut it.

obviously if you weighed your snake you have a gram scale, so just try to get rats of the appropriate size. when you buy them off a breeder, sometimes they tend to be smaller than what they say, so make sure you weigh them.