Here's what I do, and hopefully someone could tell me if I"m totally wrong with my method. I pretty much go with 1.25 qt/lb unless I have a huge grain bill and need to make room in my tun. I'll use my next brew as a sample.
26 lbs of grain x 1.25 qt/lb = 32.5 qts of water or 8.125 gallons. I make it 8.
I figure out that out of 26 lbs of grain with 8 g water added, I'll collect 5.5 g of wort. (grain absorption rate of 1 gallon per 10 lbs of grain makes me lose 2.5 gallons)
Now I take my desired post boil volume of 12 gallons and subtract 5.5 and come up with 6.5. I know I boil off 2 gallons in an hour, so I add that to the 6.5 to get to 8.5. I'll simply divide that number by 2 sparges and come up with 4.25, so I do 2 batch sparges of 4.25 gallons and wind up with 12.5 g of post boil wort. The math is rough, and some places I round up and some I round down, but it's close enough to where I don't get crazy with it. I take gravity readings after first runnings, before the boil, and after the boil. If anything doesn't seem right, I'll either add water, or add DME.
It's worked for me, and I think the batches that haven't turned out great have been other areas of my process. So is this a valid way of figuring out water quantities, or is THIS where my process has been flawed? As always, I appreciate any help anyone can offer.