Hot water volumes in a mash

I have a couple questions regarding water and mashing

When I calculate how much hot water (maybe boiling) to add to a mash to bring it up to a certain temperature, do I calculate the new volume of water based on the total mash volume in the tun or do I calculate the new volume based on the volume of water that has been used in the mash so far? I've tried the second way in my most recent decoction mash (infusion would be the same) and it didn't seem to work, but thinking about it, it seems like it should because of the high specific heat of water compared to the grain. Anyway, no matter what the physics are behind it all - what works???

My next question (this is a today-only two-for-one special) is regarding infusion mashing. When I go to beersmith to get how much water to add to what I have sitting at 122F (maybe at 1.5quarts/pound for instance), it will give me so many quarts at 167F or some other odd temperature to get it up to 153F. Why is infusion done this way? Why don't I add less water at 212 and just take the difference in the sparge? It seems to be easier to get good temp control with boiling water, so why aren't calculations done that way?

Thanks - Mark

<Bump>

Just looking for some thoughts here - Mark

To answer you second question.

You can do this by tweeking your "Water to add" number until the "Infusion Water Temp" is close to boiling.

AnOldUR said:

You can do this by tweeking your "Water to add" number until the "Infusion Water Temp" is close to boiling.

Click to expand...

Thanks for the tip and I'll do that. But why isn't it done that way normally?

OK - So the way I'm doing it, I start with 1.25 qt/pound for a 122F rest. That should be enough to mash with so I should be able to do it either way, right?