Use 1.75 qt./lb. for the grain. That would be 5.75 gal. rounded up. Sparge with as much as you need to hit your boil volume. If you're fly sparging, 15-20 min. is way too fast. If you're batch sparging, start the runoff slowly then open it up after the grainbed is set.
Is this an all grain batch? Where did you get those water amounts? Remember, you'll need water hotter than 150-155 in order to mash at 150-155. I'd recommend sparge water in the 190-200 F range.