Testing the water won't tell you how much water to change. I mean it sort of gives an idea because nitrate is one thing we are concerned about but there are other things, that contribute to poor water quality, that we don't measure.
As far as alarms go that sounds good to me. 82-84 is recommended range. I keep my tank 82.5
If you are running two heaters make sure and get an outlet splitter and plug them both into the heating plug