Use 1.25 quarts/lb = 3.5 gallons (about) for 11 lbs.
The grain will absorb about 0.1 gallons/lb = 1.1 gallons (approximate)
You also have a dead space. It depends on your equipment, but say around a pint.
Loss due to dead space and absorption = 1.1+.125 = 1.225. Lets call it 1.5 gallons.
So starting with 3.5 gallons, you get 3.5 - 1.5 = 2.0 gallons.
You are brewing 5 gallons (I would recommend 5.5 to account for transfer and trub losses to end up with 5 gallons, but lets assume 5 gallons into fermenter).
+ boil off, you need an additional gallon; estimate of 60 minute boil. Longer would be more boil off, and need more starting volume.
So for a 5 gallons into the fermenter you need 6 gallons in the kettle for a 1 hour boil. From the initial Mash you get 2 gallons, so you have to add an additional 4 gallons of sparge water to get your initial boil volume.
A number of variables: dead space, boil off rate, boil time, and volume into fermenter. All are dependent on recipe and equipment.