The tool is very simple, and completely deterministic. Just enter an Area, Thickness, etc, and hit the Calculate button. Note that if you don't enter Thickness, the tool assumes that the first field is a gross rock volume.
Geometric correction is also optional. Just enter a trap relief and select a geometry from the list. Alternatively, enter a factor directly. If you don't want to use geometric correction, just leave it blank or set it to 1.