He is adding a sparge. "few minutes while I'm heating up 3.5 gallons of sparge water to 170 in a seperate pot. " For Aussie style BIAB i would stick with the higher figure. Plan on a little longer mash (i do about 75 min) and then mash out for ten min at 170. If your preboil volume is a little high you can always just boil longer.
