eleyan, take your sweet time bringing the calcium up. Quick changes can cause irrepairable damage. The dKh is good, all you need to do right now is add a little calcium. If you also add buffer in equivalent amounts, you will not make headway with the calcium right now. Calcium and dKh work like a see-saw. As dKh rises, calcium goes down. As calcium is increased, dKh goes down. You can boost both in equal amounts (RE: kalk), but, if one is low and the other is optimal, adding in equal parts will not bring you into balance. Magnesium makes this process stabile providing its high enough. Test for that. IO in fresh mixed water runs a bit low, but, should be good to keep it in check as long as you have a source for Mg in your sand/rock/additives.
You have a 72, so in reality it might be safer and easier to balance things with partial water changes. 10% at at time, more frequently than you indicate you have been doing. If your add things and get a sudden swing back, and you can with a buffer system like this, it could really do some damage. A 300 calcium is not the end of the world or your tank life. A sudden change can be. Be thankful its taking a long time to change. That fact alone has spared you lots of grief at this point. You said you are using kalk. That helps get rid of the phosphates as well as some other things. The worst thing you can ever do with a reef tank or marine animals is to get into a hurry and try to solve perceived problems overnight. My first reaction to a problem so long as there isn't 50 gallons of water on the floor or something like that, is to chill and do my best not to react at all. Think thru a strategy to repair and research it, for maybe a week or so, and then start on a well researched plan of action that will eventually bring about results. Reef keeping: Its like the story of the tortoise and the hare.
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