Actually, I did the exact same thing you are talking about leafy on my 40g until I started dosing Kalk
. I bought the 2.5 gallon model. It took me a few days to figure out how much my tank evaporated but I finally got it tuned it. With the fan on the sump turned on, I had to drip 17-20 drops every 10 seconds to keep up with evaporation. At that rate, I had to fill it up every morning. With the sump fan turned off, my drip rate is 10 drips every ten seconds to keep up with evaporation. Big difference with the fan off. I have to fill it up every two days using this method.
Now of course days with high humidity changes everything. Evaporation slows down as the humidity level goes up. So you have to be careful "setting and forgetting" that flow valve. If it should rain, you may be overflow your sump if you drip too much.
Now I dose Kalk
, but nothing else has really changed. I am running with the fans off since Fall/Winter is beginning to set in. I am still dripping 1 drop every 10 seconds and it keep up with evaporation fine.
RR is correct in the fact that it would be so much easier to use a float valve to do this. I have just been to lazy to buy one and install it up.