I have the same setup and I use a formula to calculate the strike water temperature because there are several things which should be taken into account. The most important are the amount of grains, the ratio of water to grains and the initial temperature of the grains. And to a lesser extent the temperature of the internal walls of your mash tun (igloo). Therefor, I use a formula to calculate my strike temperature...
For instance, if you brew the exact same batch of beer, you're aiming for a 154 degree mash temp and the only difference is the temperature of your grains (let's say it's an 8 degree difference) then there would be a strike water temperature difference of approximately 1 to 2 degree depending on your grain bill.
Not a huge difference...however if you're goal is precision, you'd be much better of using a formula...below is the formula I use...
Initial Infusion Equation:
Strike Water Temperature Tw = (.2/r)(T2 - T1) + T2
Mash Infusion Equation:
Wa = (T2 - T1)(.2G + Wm)/(Tw - T2)
where:
r = The ratio of water to grain in quarts per pound.
Wa = The amount of boiling water added (in quarts).
Wm = The total amount of water in the mash (in quarts).
T1 = The initial temperature (¡F) of the mash.
T2 = The target temperature (¡F) of the mash.
Tw = The actual temperature (¡F) of the infusion water.
G = The amount of grain in the mash (in pounds).
The infusion water does not have to be boiling, a common choice is to use the sparge water at 170¡F. Then TB becomes 170 ¡F and more water (Wa) will be needed to make up the additional quantity of heat.
Example:
This example will push the envelope with three rests. We are going to mash 8 lbs. of grain through a 104 ¡F, 140 ¡F, and 158 ¡F (40, 60, and 70 ¡C) multi-rest mash schedule. For the purposes of this example, we will assume that the temperature of the dry grain is 70 ¡F (21 ¡C). The first infusion will need to take the temperature of the mash from 70 ¡F to 104 ¡F. We will start with an initial water ratio of 1 qt/lb. Using the initial infusion equation, the strike water temperature is:
Tw = (.2/r)(T2-T1) + T2= (.2/1)(104 - 70) +104 = 110.8 or 111¡F
For the second infusion, to bring the temperature to 140 ¡F, we need to use the mash infusion equation. At 1 qt/lb, Wm is 8 qt. We will assume that our boiling water for the infusions has cooled somewhat to 210 ¡F.
Wa = (T2 - T1) X (0.2G + Wm) / (Tw - T2)
Wa = (140 - 104) X (1.6 + 8) / (210 - 140)
Wa = 36 X 9.6 / 70 = 4.9 qt
For the third infusion, the total water volume is now 8 + 4.9 = 12.9 qt.
Wa = (158 - 140) X (1.6 + 12.9) / (210 - 158) Wa = 18 X 15.1 / 52 = 5.2 qt
The total volume of water required to perform this schedule is 8 + 4.9 + 5.2 = 18.1 qt, or 4.5 gallons). The final water-to-grain ratio has increased to 2.26 qt/lb (18.1 / 8).
