I've been topping off with kalk for at least 3 years now so I'll try and answer your question regarding this.
1. do i just stop dosing alk and cal?I find that the more corals and coralline I have growing the more I need to give a little extra 2 part to keep up with alk and calcium.
So,I dose about 60 ml every night of home made 2 part along with the kalk top-off
2. how well does kalkwasser keep up the levels?Well,that's all going to depend on how much evaporation you have(if topping off with kalk).And how much demand the tank has for calcium/alk.
3. and if my numbers start to fall do i have to dose extra slowly due to having kalk as my top off?I just dose to bring the numbers back up and then divide it beween the 7 days of the week.
Just be careful with alk.If this drops off while just doing kalk make sure to raise it back up very slowly.