The Atkins centre recommends aiming for 65% fat - your protein should be between 25-30% and the rest would be carbs. As for the calories, it's going to be an individual thing. I would not recommend going below 1400 calories for fear of lowering metabolism too much. Your best bet it to follow the guidelines in the book - eat until you are satisfied and when you are hungry and track the numbers for a while. If you find yourself eating too little then you can adjust to compensate by adding in more fat.

HTH
-Nat