Funny thing - as I had mentioned during my last post in this thread; I was going to preheat my plastic tun with the actual strike water and dump the grains in once the calculated strike temp had been reached - didn't work. Twice!
Two separate brew days I used this "preheat with strike water and dump grains in" method and both times I missed my mash-in temp high by right around 8 F.
Before; when I preheated the tun and dumped the preheat water, and put the grain in the empty tun and poured the strike water through a hose from the HLT onto the grain while stirring (took about 3 or 4 minutes), I was nailing my mash temps dead on.
Seems I failed to consider a couple of critical variables - specific heat of the tun, and of the grain; they respond differently based on the two different mash-in methods. Apparently, dumping the grains into the strike water, (faster process), doesn't dissipate as much heat as the other way around (slower process), and I hadn't considered that difference in rate of change.
Soooo... seeing as I am taking the specific heat equation out of the formula by preheating the tun, and then pouring the grain into the strike water, I thought I'd screw around with the calculator a bit and see what happens.
Seems if I set the Tun Specific Heat to -0.2 (used to be 0.3), I get a corresponding drop in calculated strike temp equal to about 8 F - same amount by which I was missing my mash-in on the high side, so this should give me a calculated strike temp in my mash profile that better corresponds to my system and method as it exists today.
I'm going to try this on my next brew and I'll report back.