So the cool thing about dosing with a pump, is you can get a consistent amount every day. The easiest way to do this is by creating a saturated solution of kalk. This means that no more can dissolve in it, about 2 tablespoons per gallon. This means that every time you dose say 100ml, you are getting a consistent dosage of kalk. The purpose of the stirrer is to have excess kalk at the bottom, which can be stirred in as water gets replaced.
Evaporation can change every day depending on numerous factors, so if you rely on just that you have to either deal with swings of parameters or constantly adjust the concentration.
Most people start dosing a set amount of saturated solution, then test every day for a week and increase/decrease the daily dosage until you reach consistency. Then the remainder of the evaporated water is replaced with RODI water. You can then test every few days and adjust your dosage or supplement with 2 part as needed.