Auto Dimmer Timer Adjustment

I always thought the screen dimming (halfdim) time was exactly half the time set for screensaver or screen sleep (displayslepp) - whichever was the lesser...but I could be wrong.

Unfortunately the dimming timer is not user modifiable.

The dimming timer most certainly IS user-modifiable. The problem is that it just doesn't work correctly most times. I believe this is an OS glitch as it has been a problem dating back to the Powerbook G4 days.
Go to System Preferences->Energy Saver->Options and you'll see a checkbox for "Automatically reduce the brightness of the built-in display before display sleep." (There are seperate settings for Battery and Power Adapter.) Uncheck it and problem solved, right? Well, probably not.
One workaround is to check the box (set it to dim the display) then click "Show All." (To save the settings.) Then go back and uncheck the box and "Click the lock to prevent further changes." This usually prevents dimming for me until a restart, then I have to do it again.
It's a fairly annoyng glitch, and I don't understand why it doesn't receive more attention.

The dimming timer most certainly IS user-modifiable.

All you've mentioned the the turning on and off of the timer, not the modification of the timer delay when it is on. For instance, changing it from 2 minutes (or whatever it is) to 5 minutes.