Originally Posted by Determinism
Your body will tell you. You feel much more hungry, think more about food, become lethargic, have less energy. You're just becoming a wreck.
I've never become a wreck and always had enough energy to play pick-up basketball but I've also been over 20%+ and hungry.
Then again I don't think most people have the will power to diet down to the point of destruction.
Half-way measures don't get very far. You can only do one thing well at a time. Trying to gain muscle while losing fat will result in failure.
Get to 10% bodyfat first while lifting weights heavy 3x a week with reduced volume. Two weeks maintenance. Bulk up slowly (1 lb weight gain per week max), focus on getting stronger, measure body composition changes often.