Mash with about 1.25 - 2 qts per pound of grain. If you have 6 pounds of grain, use 7.5 - 12 qts for the mash. Let's assume you use 9 qts for you mash. Your sparge will be whatever amount of water you need to make up the rest of your finishing volume, with respect to the amount of liquor that is absorbed by the grain and the amount that will be boiled off.
Absorbtion is roughly 0.125 gallons of water per pound of grain, so again assuming 6 pounds of grain, you will have about 3 qts of absorbtion (.75 gallons).
Assuming you will boil off about 0.5 gallons over 60 minutes, you need to account for this in your sparge volume. If you want 2 gallons into the fermenter, you need 2.5 gallons pre-boil.
Here's the sparge volume calculation:
9 qts (mash water) minus 3 qts (absorbtion) = 6 qts run off after the mash
2.5 gallons (your pre-boil volume) is 10 qts
Sparge water needed is 10 qts - 6 qts = 4 qts
If your boil off is higher than .5 gallons per minute, you will need more sparge water. This is my typical boil off rate when I brew on the stove, so that's what I used here.