BIABCalc is a simple app to quickly calculate how much and how hot your water should be.
With brewing in a bag, you want to mash with as much water as possible, to end up with as much wort, because you arent sparging.
Most calculators use your ideal mash thickness to calculate how much water to use- this shows you the maximum amount of water to heat up without running over once you put the grain in.
This application will also calculate how hot your strike water should be to hit your mash temperature.