[Sk8r]

If I miss your question, tell me, but it seems to me all you have to do is add kalk to that ATO bucket, and add more than you need: ONLY 2 tsp per gallon is even able to dissolve in ro/di no matter what---so you could dump a pound of Mrs. Wages Pickling Lime into that reservoir, stir, let it settle, and as water is drawn out, and more water comes in, the 'emptiness' of the entering ro/di will allow more kalk to dissolve.
This has one serious problem: you have to prop that pump up out of the white slurry of kalk at the bottom (where undissolved kalk will settle out and whence new water will find kalk to dissolve) ---and you pretty well need an Eheim pump, the ONLY one I've found that kalk won't chew up. I've had a little Eheim running in my 32 gallon reservoir for 2-3 years now.
Measuring kalk addition is not really necessary: ONLY 2 tsp per gallon of ro/di is able to dissolve [there is a trick of adding vinegar to get more to dissolve [which changes the ph---but let's not go there]. I wouldn't advise gross overdose because it's messy---but figure your evaporation rate, then dose sufficient extra kalk to handle, say, the next week, and add more kalk when you run your water tests, just part of the routine. For mine, eg, at a gallon a day evaporation, it would be 14 extra teaspoons, or a third of a cup extra kalk into my 32 gallon reservoir. If it autofilled.