I am interested in calculating the gravity of two beers from a single mash. Where by it tells me the estimated gravity of the first runnings, and then the gravity of the second runnings after sparging.
There's a calculator for that here as well as an interesting article on it. There's also Randy Mosher's take on it here. To keep it simple, you'll get something like 2/3 of the wort sugars from the first runnings and 1/3 from the sparge, as long as you plan for half the water in each one. It's all very easy really.