I agree with PhilE, you can achieve this for X via the xset utility.
Precondition is a working backlight sysfs node ... look into /sys/class/backlight if there is some node (but I guess so, as you write you can dim your display fine)
I have a similar setup - a touch display where the backlight shall be switched off completely after some time.
I use following command to setup a screen blanking after 1 minute timeout:
or, if you like to do it over ssh...:
After 60 seconds, the screen turns blank ... If i touch the display, it gets lit again.
Now, for your setup, you probably have to setup xscreensaver. I bet there is at least one screensaver which dims the screen to some defined value.
Activate that, and then run the command I wrote above ... theoretically, it should then dim after 60 seconds.
Hope that helps.