Well, if you're doing a regular 1.25 qt/lb then you would mash in with 2.34 gallons. You can also use the general grain absortion of .12 gallons/lb giving you about a gallon of water lost to grain absorption. That means 1.34 gallons from the first runnings.
Sparge depends on your boil off rate. I have a very high boil off rate. So my preboil volume is 7 gallons. I think most people get away with 1.5 or so gallons per hour. We'll go with 1.5 for this example. So take your preboil volume and subtract your first runnings from it. (6.5 - 1.34 = 5.16) So sparge with 5.16 gallons of water. You shouldn't get any grain absorption when you sparge because they're already soaked, so you just get back out what you put in.
That should get you in the ballpark ... i think.