I did this one a few weeks ago. Mashed-in with 3.5 gallons of water. After the 1 hr mash, I added 1.5 gallons of water at 180, stirrred and drained. I then added another 3.5 gallons, stirred, let sit for 10 min, drained. My pre-boil was about 6.8 gallons and I boiled off about a 1 gallon in a 10 gal Mega-Pot. 5 gallons exactly went into fermenter. I didn't dump the gunk at the bottom of the pot. Seemed to work out well for my first AG but my initial strike temp was a tad high (grain was 77, strike was 168, should have been 166). Good luck.