Are you talking about subzero mode level, jmejiaa?
It's in the settings (gearing icon on the lower right of the widget): Parameters>Brightness>Backlight zero level (well, sorta: translated from my French interface)
The higher you'll set it, the brighter it'll start cutting down on backlight power supply.
Else, for more classic brightness setup (dark filter over the display), you go to Installation>Profile editor, and set your "luminous flux on the sensor/brightness of the screen" function (with two points only, it seems to be some sort of S curve).
not sure if I understand your problem correctly, but if you have set brightness levels for two separate lux values, then brightness for lux values between those two is calculated/extrapolated automatically - moreover there are three different methods for this, you can choose one best suiting your needs in the settings
If I leave it in auto, does it switch between day/night profiles? I don't really need multiple profiles but I don't see a way to modify the values for "auto".
I just tried modifying the outer bounds for both lux/lum and it seems to do what I want..