StinkyVp, I do BIAB. I use the 1.25qt/lb ratio, so with 6lbs of grain or so, you're talking about roughly 7.5qts or 1.875g of mash water. Then I sparge with however much water I need for the preboil volume to be 6.5g. So if I assume .5g of liqiud lost in the grain (it's usually not this high, but just to make it easy), I need 6.5g - (1.875g-.5g), or 5.125g of sparge water.
That said, boiling this for 90 minutes could reduce the volume a bit more, so take some good notes the first time and adjust as needed.
That said, boiling this for 90 minutes could reduce the volume a bit more, so take some good notes the first time and adjust as needed.