Something like that or the ability to control brightness with Flux would be handy.
I use a Macbook Pro with an external monitor.
El Capitan does not allow any adjustment of external monitor's brightness.
I used a 3rd party app called Brightness Control, it's able to adjust brightness across both screens by creating a translucent mask and varying its opacity.
HOWEVER, Brightness Control and Flux do not work together. So I can either dim my display and lose the color temperature control, or vice versa.
Since Flux controls color temperature, it would be handy for Flux to control brightness too.