I use two...
Screen Filter (widget) that you set to a pre-set %. It just overlays a darker screen, does not actually dim the screen.
Night Filter to darken the screen, and overlay red to better try to keep my night vision.
Both are nice, but I don't rely on auto-dim - most situations for me are different, and I use a different level.
You'll have to try a bunch until you find the one that suits your needs. Cheers.