I started by taking the "recommended" amount of food, by either weight or caloric content (generally a balance of both). I ALWAYS brought food back, and generally a good deal of food for a short trip (we're talking nearly a pound of food for 2-3 nights). I've developed what works for me, it's plenty of calories to keep me going strong, and plenty of bulk to keep me from feeling hungry, and it's generally below 2,000 kcal per day. Doesn't really make sense, I'm an average sized dude, 5' 11" 165#, but it works for me. I've never lost any real notable weight while on the trail either.

Just have to play with it til you find out what works best for you...
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Light, Cheap, Durable...
pick two