I cant see having a tank without an auto-top off.
Again, theres many ways you can do it. For years, I didnt use a resevoir. I have the rodi unit hooked straight to a float valve in my sump. Very simple and never failed in any way. My current setup is still under construction. I will have a 20 gallon reservoir where the rodi will goto a float valve. I will also have the float valve goto the sump. From the resevoir, I will add kalk or salt or anything I ever need and have a dosing pump slowly pump this into my sump. I will keep the dosing pump set to a speed slower then my evaporation rate, and the rodi will match whatever more water is required to reach the correct water level. Ive never really felt the need to use a float switch (electronic) for auito-topoff. But I do use one in a 5 gallon bucket that catches my skimmer water that goes through the collection cup. When the bucket is full, the switch shuts off the skimmer. Since the bucket is 5 gallons, I know exactly how much salt I need to put back in the system.
If I had more room..I would have another container such as a 50 gallon drum. Have this filled with salt water. And have two pumps on timers go on for a small amount a day. One pump to take water out of the main tank and one pump to pump new salt water from the drum to the tank. It would be like an automatic mini water changes!
cuttlegirl-DIY float switches arent too hard, But you can also just buy ones from autotopoff.com. I believe thales suggested that site to me a whole back.