My potable 45 gallon tank sensor reads "Full/Half/Empty" which corresponds,
approximatly, to 35/20/5 gallons. When the sensor reads empty I have 5 *usable" gallons remaining. When the tank is
actually empty, *then* I suck air.
Depending where the potable water out-flow pipe on your tank is located (Mine is near the bottom on the sidewall.) is where the water will stop being pulled into your system.