To add to Rizzo's advice, I've read that for the block calculation, a growing teen should use a higher activity level than their actual level, in order to provide the additional protein necessary for growing. Another way to accomplish the same thing would be to use you actual activity level and add a couple blocks to the result. In addition, due to your low body fat %, you should double (or, if your activity level is very high, triple) the amount of monounsaturated fat blocks you eat in each meal and snack. If you double the fat a 4 block meal of 4P, 4C, 4F would become 4P, 4C, 8F. Triple fat for a 4 block meal would be 4P, 4C, 12F. A one block snack doubled is 1P,1C, 2F, a 2 block snack doubled is 2P, 2C, 4F, and so on.