I never really put much thought into calculating sparge volume. Software just makes it too easy. If I'm starting out with 7 gallons pre-boil and I only get two gallons from my first runnings, then I know I need to collect 5 more. The grain isn't going to absorb more water after the first time around, so using Bobby M's double batch sparge I use 2 equal infusions of 2.5 gallons. Once you have your brewing efficiency set, it works just about every time.
I guess if you don't have software, you'd have to measure your gravity to know when to stop sparging. The only problem I see with this is that you might get stop when your gravity reaches 1.010, but if your efficiency was better or worse you would end up with more or less wort for the boil.
I probably didn't help you in the least bit, but I'm curious to see if there is a way to figure this all out with software and still get your desired boil volume.