Measuring Specific Heat

You can find the exact specific heat of your mashtun with the Mash Adjust tool and a floating thermometer.

Accurately weigh your mashtun.
Accurately measure out 3 gallons of water.
Heat the water to 170 (actually, any temperature will do, but this is closer to brewing).

In the tool:
Starting Water volume: 0
Weight of grain: 0.0001
Starting mash temperature: ambient temperature of the room
Weight of mashtun: as weighed
Mashtun Specific Heat: 0
Water Addition Temp: 170 or the exact temperature you use.
The mash water ratio will read something crazy. Ignore that.

Quickly add the heated water and floating thermometer to the mashtun. Cover and wait 10 minutes.
Read the water temperature.
Input the water temperature as "Target Mash Temperature" in the tool.

Now, simply change the Mashtun Specific heat until the "Water Volume to Add" matches 3 gallons.
That's your mashtun's specific heat!

If you are changing Specific Heat in the Profile tab, it won't effect the recipe until you re add the profile to the recipe. Otherwise, changing the profile within the recipe (click the red check box next to the profile name) is what you want to do. You also have to have the Adjust Temp for Equip box checked in the mash tab.