Yes, this is pretty much what I'm getting at. There are a few gallons of water left in my HLT after sparging, along with some amount left in the MLT. I do not treat my sparge water at all, but I do treat the mash water. The reason for this, is that my water isn't far off at all for most styles of beer that I brew. So, typically, I just need to lower the pH a bit, and add some sulfate. Since sulfate helps lower the mash pH, I just put it all in the mash, so I get my target pH, and the diluted result in the BK with the desired mineral content. For this reason, I've been putting the [pre-boil volume - mash water volume] for the sparge value, and unchecking the box that says "adjusting sparge water." So, the way understood it, that value is used to calculate the diluted result after sparging.
Am I interpreting this correctly?
Thanks again for the help!