For 5 gallon batches, I just use 5 gallons of strike water for the mash. Once I drain the mash tun, I see how much wort I got and add enough sparge water to get my pre-boil amount. So, If I'm shooting for 6.5 gallons pre-boil and I get 3.5 gallons of wort out of the mash (due to grain absorption), I just add 3 gallons of sparge water and drain. Voila!