I was going to write a little rant about how this should be possible without 3rd party software. Researching a bit, I found that Apple has restored the ⌥+⇧ modifier for volume as well as for brightness (if that was even there before).
So no need to install this; adjust brightness with ⌥+⇧+☼ instead!
http://apple.stackexchange.com/questions/19146/is-fine-volum...
Edit: Trying it out, it seems the actual brightness level has the same minimum as before, even when the setting is at 1/4 of a square. It only works for values higher than one square. :(