The old fashioned sensors in my unit just do colors......green, yellow, orange, red. When I read of the problems with the newer sensors, I am glad.
I can visually check how much fresh water I have, by looking in the lower galley cabinet, which is the best indicator of how quickly I need to fill the tank. The sensor on the fresh water will be red when there is 1/3 tank of water left....so I just ignore the sensor and pay attention to the water level.
You should know in gallons how much your grey tank will hold, and go from there. Mine holds 10 gallons.
If you are not using a dishpan, this is a good way to keep from filling your grey tank quickly. Use just what water is needed, then dump it into the toilet or away from your site under a tree or bush, if that is permitted.
The dishwater is good for sloshing around and breaking up the contents of your black tank.
Maggie