I agree with Tellie. I think you are getting unnecessarily complicated. If you have an accurate measure of how much you use in a given time, and if your watermaker has a reasonably constant production rate, you should be able to make the necessary calculation easily enough without any instrumentation. I usually don't start my water maker until I have used a third or more of my tank capacity. Unless you are worried about keeping trimmed to the design waterline, or have stability issues which necessitates keeping the tank full at all times, I don't see any pressing need to keep it constantly topped off.
'Tis evening on the moorland free,The starlit wave is still: Home is the sailor from the sea, The hunter from the hill.