The easiest way I found to set my brightness was to use the xbacklight package:

sudo apt-get install xbacklight

Now you can set the brightness easily to a certain percentage via the command line: xbacklight -set 50

To set the brightness automatically when I log in I use “Startup Applications” like so:

• Hit the super key and search for “startup” and open the “Startup Applications” app. Once opened, add a new item:
  
• Name: Set Brightness
• Command: xbacklight -set 50
• Comment: xbacklight

You can obviously add whatever you want in the Name and Comment fields. Then you should be good to go.