I use 1.5 quarts of water per pound of grain as I can do the math in my head. I brew 10 gallon batches.

Say I am using 20 lbs. of malt. Heat up 30 qts aka 7.5 gallons of strike water. Malt absorbs 0.5 qt/lb., so I will collect 20 qts or 5 gallons. I want 13 gallons for the boil, so I heat 8 gallons of water for the batch sparge addition. It keeps things simple.