Being basic, Kalkwasser in my ATO is causing my tank's pH to rise. Should I switch to water once the pH gets to 8.5, and then back to Kalkwasser when the pH is ~8.2?
Evaporation is an important thing to know. Kalk is typically dosed at night to repl evaporated water and to largely eliminate the natural pH drop during darkness.
If you add kalk to your ATO and your pH is going very high you will need to reduce the amount of kalk you are adding, or only use your ATO at night. A tablespoon for every 5 gallons would be a good starting point.
How much kalk powder are you adding to your top off and how many gallons is it? Depending on how much you might be able to scale it down and not have to worry about switching. Its recommended to start with 1/2 tsp per gallon of RODI and adjust if needed.