I would not use the ph probe to control your effuent ph, but maybe as a safety feature in case your effluent ph gets too low. If (and that's kind of a big if) you have a reliable way of controlling the CO2 and effluent flow, you should have a very stable effluent ph. Shutting the CO2 flow on and off sometimes makes it more difficult to keep a steady reactor ph, and sometimes the flow varies when it comes back on. You could set it really low, like 6.4 or something.

Effluent dkh is usually very high when the media is pretty new, but will eventually drop to between 20-30 dkh after the initial surface of the media has dissolved. I think this is probably due to the way alk test kits work; they will get skewed if there is any dissolved or particulate aragonite in the sample. What your actual dkh is with regards to ph and effluent flow is dependent on your reactor size, efficiency of circulation, and the ph. ARM seems to have a "sweet spot" around 6.65-6.7.