I don't worry about trying to precalculate my exact sparge water volumes. I know that I boil off around 3/4 gal during an hour boil. I also figure in 1/4 gal loss to trub/hop absorption (whole hops). If the recipe with a huge amount of whole hops, I'll add an additional 1/4 gal to that. I normally shoot for a preboil volume of 6 gal.
I always mash with 1.25qt per lb of grain. I figure I'll lose approx .1 gal per lb due to grain absorption, but I don't usually even bother calculating that.
I have a yardstick with gallon marks that I can stick in my boil pot to measure volume. After I drain my mash runnings into my boil pot, I measure my volume. If I've got 2 gal in there, then I need 4 gal of sparge water. I'll usually add an extra gal to what I'm heating up, so I have it ready i case I need it.
If your beers are too weak, then it sounds to me like you're just not calculating your recipes properly. I'd recommend using some software, like Beersmith, ProMash or BeerAlchemy (mac). You can plug in your desired preboil and post boil volumes, and then adjust your grain bill till you hit your target gravity. I use BeerAlchemy and am usually within a couple points every time. There is also a decent recipe calculator at tastybrew.com that is free to use.