I'm still pretty new to home brewing but here's how I do it. I heat my strike water to about 160 and mix in grain. I wait and hour and collect my first runnings. I then heat water to 170 and mix in. Let it sit for 15 mins and collect the wort. My question is If I collect 3.5 gallons off my first runnings, how much water do I need on my second in order to collect 6.5 to 7 gallons of wort. For my mash I generally follow a two to one ratio. For ten lbs. of grain I use 20 quarts of water.