The normal resting level for a septic tank is around 85% full which typically means the liquid is about 9 inches below the "ceiling" of the tank. Specifically the level corresponding tot he lower edge of the outlet pipe.
It works on a one gallon in (from the house), one gallon out (to the leach field) basis.
(One morning on WRKO Radio) The tub's not clean! Let's remodel the bathroom.
(One afternoon on WRKO Radio) It's time for an oil change. I'm going to trade in the car for next year's model.