Is it possible to work out how much a given cake, pie, etc will weigh and how much fat, salt etc it’ll contain after it’s made?
There are various websites that will do that for you. Here’s one.
Is there anything which can do something similar for weight?
Caloriecount.about.com will allow you to use weight for ingredient input. You have to figure out “servings” on your own.
As for final weight, no. I think there are too many variables for that to be at all accurate. Variables include oven temperature, duration of time cooked, and relative humidity to name a few.
Best bet would be to make the item in your kitchen 3-4 times and use an average.
Unlike most foodstuffs, tarts generally taste worse when they weigh more.
I was afraid of that. :o