Ms Missy, There is not a chart as far as " if your snake is 300 grams then feed him a 30 gram rat as far as I know , because everyones snakes are different but as rule of thumb I believe its 10-15% of your snakes weight. I personally use the girth method. I feed my ball pythons rats that are about as round as the thickest part of their bodies and it has worked great for me.