I should chime in here, since I set up my tank for ATO with the Apex....but I'm sort of out of the box on it. My RODI feeds a small rubbermaid res that is above my sump on a shelf. The RODI feed goes to a float valve (one of those push up to stop the flow thingies.) at the top of the res. Near the bottom of the res is a float valve tied to the apex.
At the bottom of the res there is a bulkhead that feeds into a valve (so I can shut it off instead of draining it), followed by a splitter. One side of the splitter feeds my Calc reactor, and one side feeds another float valve that is in the sump.
Some people feed the RODI directly to a float valve in the sump, but this isn't a good idea. RODI units don't work well with small amounts of water, and if straight to the float valve it will replace evaporated water smidgen by smidgen.
So, my version works like this:
if the float vavle on the bottom of the res tank drops, it sends an email alarm and an audible alarm on the base unit that won't go off at night (because freshwater res low isn't an emergency). I then go turn on the RODI, it fills the res to that float vavle, which turns the RODI off. I can now wander about while its filling without worrying if it will overflow when I space it. The res float valve and the calc reactor have gravity fed freshwater at all times.
The freshwater res is the blue thing on the shelf.
the white thing in the lower right is a thermoplastic mount I made and then siliconed into the sump. It holds the float valve at the level I want in my sump, and also has a float switch higher up to warn me if the sump is high.
With the cost of fittings, res, switches, breakout box for the apex, it was about$175, plus a ton of work. If I had to do it over again, I would just buy an ATO.