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.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?