Just to ad my 2c worth,when I pb/pm biab,I nash 5-6lbs of grain in 2 gallons of water @ 153-156F. Then sparge with 1.5 gallons of 170F water,all spring water. This gives about 3.5 gallons boil volume in myr 5 gallon BK/MT. I do wind up topping off with 1.5-2 gallons of the chilled spring water to get up to recipe volume. If your mash goes well & you sparge enough & just barely get a nice boil (not too violent),then you will get minimal boil off. I chill the wort down to about 75F,then after straining to get gunk out & aerate into the fermenter,I use spring water chilled in the fridge a couple days to top off. This gets the wort down to at least 65F,if not much lower. When done in conjunction with Irish moss or the like,it gives a nice cold break too.