KOH Freezing Temps
The temperatures at which various KOH concentrations will freeze can be found in the charts below. One chart is for Fahrenheit and the other is for Centigrade.
As an example, lets say we want a 20% solution. You would use 200 grams of KOH with 800 ml of water. 1 ml of water = 1 gram. This would make a 20% solution of KOH by weight. That's how the charts below are calculated.
Note: These charts show the freezing temps down to a theoretical maximum. In actual practice you will find that you don't get much benefit going higher than 28%. The reason is that the solution becomes saturated with chemical, and after that point, no further chemical will dissolve. That means that more chemical will just be floating in your electrolyte, clogging up any check valves or small orifices. The amount of chemical that will dissolve increases with increases in temperature. Therefore at very low temps, less chemical will dissolve and therefore provide less protection from freezing. So above 28% you really don't gain anything.
Also realize that as the water is consumed making HHO, the concentration of the remaining electrolyte gets stronger. When you use 1/2 the water, the concentration will be double. This may get to the point where there are so many KOH solids in your mix that your system can get clogged up. So when you are using heavy concentrations like this, you should keep your reservoir topped up more often.