There is no formula other then feed em regurlarly and see what happens. You cant know what percentage of food turns into body mass. Its the same in all living creatures. Its all about metabolism. Some animals have higher and lower metabolisms. Just cause you pound food doesnt mean anything. The animal will grown at the rate its metabolism will allow it to. Also dont be so concerned about the so called magic weight of 1200 grams. Just cause they get to size doesnt mean they will breed. Yes you want at least 1200 grams and body mass for developing and carrying and laying but if the animal is sexualky mature then it means nothing. Studies have been done on ball pythons sexual maturity and an average sexual mature girl hits it at 3 years old. Now some will hit sooner but the rule of thumb is 2 1/2 to 3 years of age. I have a mojave that is sitting at close to 1200 grams and she is only 14 months old. Theres no way im breeding her till at least december of 2012. I also have a couple normal girls and a spider girl that are over the 3 year mark and they are 1200 to 1300 grams bit girthy so i will try them this year but focus more on sexual maturity and not weight