How Does Freeze-Drying work? (The Physics of Making Freeze-Dried Backpacking Food)

Thanks, I am about to buy a freeze drier after following the articles on BPL.  One thing I have not figured out is there a way (not trial and error) to know how much water to add to properly rehydrate your food?  Can you weigh the food before and after freeze drying and get an accurate amount?

Weighing before and after will get you the information you need. Just remember to convert units of mass to units of volume (easily done if you use grams and milliliters) unless you plan to bring a scale with you on the trail.