I started out just using numbers that were reasonable based on reading then kept track of how those numbers work in real life on my equipment. So you can start with say 1.5 qts per pound for your mash, 1 gal per hour on your boil, multiply .1 times pounds of grain for gallons of water absorbed and figure 1/2 gal of trub. When you have total water amount of water just subtract what you figure for your mash and that's your sparge.

Numbers like this should get you close then keep track of what happens next time. the time after that etc. and then you'll have your numbers for your set up.