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.
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.