Personally I would mash that with 1.5 gallons (1.5 quarts per pound). You could do 2 quarts/pound, which gives you the 2 gallons. I'm not the expert on mash dynamics, but 3.5 gals sounds like too much water. Do you have an instant read probe thermometer (for cooking) that you could use? That's what I use, they are relatively cheap.