Thanks. I hate it.

Instead…

1. Have a shaded zone at the left of the brightness bar (which most people will have auto-managed).
2. If the user drags into this zone auto-brightness is suppressed and the user can select an amount of extra dim.
3. If the user drags out of this zone, and auto-brightness is enabled, allow to take over. Otherwise use the brightness at the users point of release.

No need for a magic setting. Brightness control is all in one place.