[untamed]

You need to slow down that evaporative replacement system. I think you are saying that you have a MJ1200 triggered by a float switch..and that the float switch doesn't trigger until the tank is about 1 litre low...then it dumps in 1 litre really quickly.

What I would recommend is that you pick up some irrigation drippers and put 2 or 3 or 4 of them onto the output of the MJ1200. The float switch will trigger the pump, but it will then take the pump a relatively LONG time to pump in the replacement water because it is pushing that water through the drippers.

This solves a few issues for you.
1) it prevents Kalk from going in too fast (your original concern)
2) If ever anything goes wrong with the evap replacement system, nothing can go wrong very quickly. (ask yourself how much time you would have to react if the float switch stuck ON in your present situation)

I have some concerns about doing what you are thinking, though.
1) Kalk will tend to clog up your drippers (if you take my advice on the drippers)
2) Kalk will be really hard on your MJ1200.
3) You lose control over how much Kalk you are adding because it is now linked 100% to evaporation. Somedays it will be more, somedays it will be less. On a small system your Ca/Alk levels will be erratic.