Originally Posted by 73Drvr
I believe the variable changes for the second infusion. In this case you use boiling(212 deg) water and the variable is the volume required to raise the mash temp up to your desired temp.
I agree with this, and you also need to take account of the 20 qt of 156F water that was added at dough in. Your formula does not take this into account. Also, to get accurate results, you need to take the thermal mass of the mash tun into account. I suspect your constant of 0.05 does this to a certain extent, but that would not be accurate if you changed the grain bill.