Well, sticking with a fixed mash ratio means that its thickness/thinness doesn't change either.
11lbs of grain x 1.25 = 13.75 quarts or about 3.5 gallons.
Grain absorption is about half a quart per pound so you'll lose about 5.5 quarts. When you drain your first runnings, you'll get about 2 gallons out.
If you want 6.25 gallons preboil, you'll need to sparge with about 4.25 gallons. Then again, you never mentioned what your batch size is, how big your kettle is, etc.