Similarly to the system volume dialog, you can change the output volume by a slider (see the red box). The currently set value is displayed as a percentage of the original maximal volume, next to the speaker icon (green box).

The slider also indicates original 100% position (red box). You can change the maximum of the slider (blue box) either in the settings (pink boxes) or by pressing the full range button (green box)

The app provides two modes: a normal mode with selectable max gain or a full mode with 999% maximum gain.

Details of the normal-mode ranges can be found here.