After glancing through a manual I found online, I would guess that the backside dial is not connected to anything electrical, and only works as a mechanical calculator for the unit's two settings:
- Auto, for constant measured light at the sensor
- Manual, for constant (full) output of the flash
But you have a digital camera, you can try it anyway!
If you only want a darker photo, use a smaller aperture or lower ISO.
If you want to balance the flash with existing light, or use a larger aperture, you can waste the flash output somehow:
- Bounce the flash from the ceiling or walls
- Make a DIY softbox or diffuser from a white plastic container
- (Why don't you want to take the flash further from the subject? You want to hand hold both the camera and the flash?)