Can I configure the relative brightness level that auto-adjust will set?
You can use Shady.
The way it works is that it...

...pull(s) a virtual shade over your screen ... dimming the display to a more comfortable level.
Shady will save your shade level and restore it next time you launch the app.

As it's just a virtual (but effective) dimming method, I doubt that this saves much energy, but I cannot confirm this as LED displays should consume less power if displaying darker colors.