How's this sound for a ATO?

[lionhead]

I suggest going with a bigger tank or barrel for your top off water.

[ReeferRob]

Well everyone pretty much covered what I was gonna say LOL, but ya get the RKL float switch kit and run her that way! While your at it you could grab a third and do a float switch for your skimmer like I did!
Up sizing the ATO resy is a good idea too. I use and old salt bucket down in the basement on the grow out tank and I have to re fill it almost every night almost defeating the purpose.

[rmalone]

Im just reiterating what was said. You must use a float system, evaporation isn't really a constant. that can be controlled via a timer, like you could 2-part. You could use the rkl as a fail safe so a screwed up float wont empty you ro reservoir, that would pretty much be like using a JBJ ato.