I've been thinkin' about this one, and I'm not sure I have an answer, but I'll bite!
First of all, it's not an exact science. If it were, we wouldn't all be here.
Example: Let's just say a gallon of water weights 5 pounds. If you were to drink a gallon and not use the little girls room, you could be 5 pounds heavier w/o actually gaining any weight or consuming ANY calories.
Example: If you were to eat a snicker's bar (250 cal) while walking on a treadmill (for 250 cal) would it be a wash? No, because your body uses the calories (fat/carb/prot) in different ways.

An added real pound (whether it be fat or muscle) of body weight occures when you retain +or- 3, 500 calories.
I believe this to be a guideline, because I think many here have proven this wrong.

Good luck, HTH,
Peace,
Bren