Hello, this is Betta's husband. She asked if I could answer this one for you.
You are on the right track. Keep using the Purple Up (great stuff!) to bring up the calcium and maintain the carbonates. Once the calcium gets up above 400ppm you can go ahead and start dripping Kalkwasser. The Kalk will help keep up the caclium levels, but may not keep up the carbonates. It will depend on a lot of different aspects of the aquariums chemistry. Keep an eye on the carbonates to make sure that they do not start to drop. If they do, then simply add carbonate buffers to keep the dKH up where you want it. Don't try to keep it strictly at 11 dKH as it will fluctuate. As long as you can keep it between 9 and 12 dKH you will be fine. If you're dripping the kalk then dilute the carbonate buffer with aquarium water before adding it to your sump or tank (for example, you can take about a gallon of water from the tank and mix the buffer into it, stir it up and add it to the system).
You may notice that your coraline algae will start to grow quickly.
When it does, keep an eye on your magnesium levels as well. If the magnesium levels drop below 1280ppm it will impede the solubility of calcium and you may have problems keeping the calcium up where you want it.