I would assume that since you used 10% per month as an example that L.West meant 15% per month. 15 goes into 100 6.666666 etc times. So if an adult ball python should eat 15% of body weight per month and she weighs 2500 grams then she should be eating 375 grams of rat per month. This would be 187.5 grams every two weeks. Since large rats are 175-275 grams you should feed her a large rat on the smaller side. Or a rat that is as big around as the biggest part of her body which is what everyone I know uses. For an adult female ball python this would be a large rat every two weeks. Up until they reach adult size you should feed a ball python every week, only adults should eat every two weeks.