The dkH of the effluent will depend on two factors, one being the CO2 infusion rate and the other being the effluent flow rate.
I don't really check my effluent's pH or dkH any more. I just test the system and see if it's keeping up.
When I first started using a Ca rxr, I set my bubble rate to maintain an effluent pH of 6.8. I'd adjust the CO2 to raise or lower the effluent pH. Now I just eyeball it and if the Ca rxr isn't keeping up, I increase the effluent flow rate but leave the bubble rate alone. It seems that once things started "clicking" I don't have to worry about it too much.
Starting out, I'd shoot for an effluent pH of 6.8. Test the dkH of the effluent but don't be obsessed with it. Be more concerned with how your system's alkalinity and pH is reacting to your settings and take it from there.
I've tested my effluent on a few occasions and have had alk of anywhere from 9 to 15 meq/L (roughly 24-40 dkH), but I don't test it regularly or shoot for any particular figure. I find it works best if you let your tank tell you what it needs.
My practice is that if the system pH is too low, I reduce the bubble rate and if the system's alk is too low, I increase the effluent flow rate. Not very scientific at all and chemically it doesn't make a whole lot of sense, but it seems to work for me. I also do an occasional pH test of the effluent just for grins, but I mostly just tweak the reactor based on the levels in the tank.
HTH
