I had the same needs and was previously using the script in this thread to reduces screen brightness to minimum.
Recently I discovered and started using Light. I set some shortcuts using the alt key with the screen brightness keys which works well for me. Pictured is my minimum brightness shortcut and brightness step up shortcut.
An increase brightness value of 0.18 gives me 4 steps between absolute minimum (0.1 in Light) and minimum brightness using keyboard controls (1.0 in Light).
You can also turn the backlight off completely if you keep stepping the brightness down or set it to zero.