I have always been very nervous about adding kalk as part of your top off scheme. The chances for overdose are just too high. I think the best was not to have an overdose is to avoid this practice in the first place. I think the kalk should be added in an overflow style system using a dosing pump.
I have outlined they way I set this up here:
https://drive.google.com/file/d/0B5QJJLprX7aQclZsN2pyLU1zTU0/view
Hope this saves someone from a crash,
Bruce