Like a lot of other users, I have problem with the brightness settings on my laptop. But there is
xbacklight which works like a charm.
How do I bind
xbacklight -dec 10 to Fn+Up?
I have tried to use custom shortcut with no success.
According with this post, in general there is no way to know that Fn was pressed and the keycode of key-combinations including Fn depends on the individual keyboard design. In fact, when you press only the the Fn key, this doesn't send any data to the computer.
So unfortunately, you can't bind this combination: Fn+Up. More about: Fn key.
What you can do is to use another key combination to add a custom keyboard shortcut.
You can use a software called "Autokey": it will start after the login if you want and supports scripting & key combinations.
You can install it from the Ubuntu repository (
apt-get install autokey).