0 to 8 is your gain setting. in a perfect world you would match that number to the preout voltage of your headunit.
ie, if you headunit has 4.5 volts preout then you would set the gain to 4.5 volts. but other factors come into play such as voltage loss through rca's, not all songs have the same level in them, etc, etc etc.
anyway the best way to do it is with an O-scope or at least with a DMM if you have one available.