One (or two for redundancy) wired normaly open at the top of the tank that activates when the water level drops. Instead of relying on volume and pressure the switch would activate simply upon water heighth. Auto top offs too!
Then possibly you could use your pressure switch on a timer that would drain water to a certain level. As long as you could get water out of the tank faster than you could get it in you wouldnt be wasting too much water.