Go download some brewing software like BrewTarget (Free) or some of th eother s(not free).
They'll help you with those calculations, and much more.
You start with your yield, then add for boil-off, then add for grain absorbtion. You'll also want to figure hot much loss you get due to spillage, liquid left in hoses, at the bottoms of your vessles, etc.
- Brewin' and 'Quein' since last Tuesday.Bottling the Belgian: A Photo Odyssey
Beer is the mind-killer. Beer is the little death that brings total obliteration. I will face my beer. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see it's path. When the beer has gone there will be nothing. Only I will remain.