Matt, my Kalk doser is my top-off and it is working great. Well it was until the tube cracked around the uni-seals :( Dan's gonna build me a rectangular reactor to replace it soon.

Anyway, My top-off controller turns on a 110v solenoid which feeds the Kalk reactor (sealed), which forces clear/saturated water into the sump. When the sump is full, the solenoid closes and the mixing pump comes on for 15 minutes, then settles for the next cycle. I use two float switches (Min/Max) so that I get about 3-5g of topoff at a time. In my system that cycle probably runs about once every 6-8 hours.

I am very pleased with this setup. I use a PW40 as the mixing pump, and used one of these and a couple of these to build the control/timer circuit.