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.
Tornado victims: Do not rush to rebuild. Take your time and look for and get a good contractor. Or consider selling the property and moving to a home that is ready to live in.