Sounds pretty good to me. I get nearly 2 gallons of evaporation on my 72G daily, with fans. (The drier air of winter w/ gas heat has sped up my evaporation rate.)
that all really depends on how much water movment you have, temp of tank vs. room temp. I'd say mine evaps around that....maybe more. I dont know now that I think of it, lol. Perhaps I should find that out.
I will pay closer attention. I work from home and add fresh RO/DI manually whenever I see it has lost some. Maybe I am putting in more than I think. Will measure tomorrow. Seems like I am having to make a lot more since I upgraded.
If you build some fail safe protection, you can limit the damage. My auto top off runs on a timer that drives a inline solenoid for the auto top off. You can find all you need to do this on the web.
You can get an ATO with 2 float switches and a use a 5 gallon bucket. Then you only got to worry about 5 gallons of water not a stream from the RO/DI unit.
The main benefit to an ATO in my book is the salinty never changes. It stays constant, which stability is the most important factor in marine aquaria.
Everyone knows I am a CHEAP man... but I hooked up a JBJ ATO unit yesterday it was a) easy as pie. Only took me about 5 min to hook up. and b) something I should have done from the start!!