A recent zymurgy had a table about the expected gravity of first runnings vs the mash ratio. I do parti-gyles for all of my big beers and use only the first runnings for the big beer. The thicker the mash, the higher the gravity (and less runnings per pound).
1.25 qt/lb will get first runnings close to 1.090, 1.1 upper 90's, 1.7 in the 70's.
You have to figure out the amount of grain backwards. What pre-boil gravity do you want? Let's say 1.090 . So we will mash at 1.25 qt/lb. (Or check the zymurgy chart.) Because of grain absorption (let's say 0.5 qtlb) you will only get 0.75 qt of first runnings per lb of grain. What pre-boil volume do you want? Let's say 6.5 gallons (or 26 quarts). To figure out the amount of grain, divide 26 by 0.75 and you'll require 34.66 lb of grain. Mash 34.66 lb of grain with 1.25 qt/lb (mash volume) = 43.3 qt of water (= 10.83 gallons). Add the volume of wet grain 34.66/3qt = 11.5 qt or 2.9 gal for a total mash volume of 10.83 + 2.9 = 13.7 gallons. Got a big mashtun?
Simply run off the first runnings (6.5 gallons) into the boil kettle. Then batch sparge with 6.5 gallons and collect 6.5 gallons of second runnings. In my experience the gravity of the second runnings will be about half of the gravity of the first runnings or ~ 1.045. If you have a second boil kettle you can have the 2nd beer boiling within 15 minutes of the first.