How about providing a calulation where you input the current water volume and temperature, and then you can either:
1) Input the temperature you want to get to, and the temperature of the water you are going to add, and it works out the volume of water you need to add.
2) Input the temperature you want to get to, and the volume of the water you are going to add, and it works out the temperature of the water you need to add.
This can be used for a few things, like how much boiling water to add to your sparge water to get it up to temperature, or when doing an iterative mash, what temperature to do the first sparge to bring the water down from the first mash out temperature to get it down to the second mash out temperature.
Cheers Tim