It depends on what target number you're talking about.
Are you talking about the Target number for the RPG?
All Target Numbers are fixed in the RPG. You simply add your skill, your link attribute bonus and any other modifiers to the roll.
So for gunnery, your target number is 8. When you roll your dice, you'll roll 2d6 then add your skill (6) to the roll+link attribute (1)+Any other modifiers (range, night time, obstructions, etc).
You may be thinking "Hey, with a 6 skill and a +1 link attribute for +7 to all my rolls, and 2 being the lowest possible roll on 2d6, I'll always reach a TN of 8!" Well, it seems that way, but keep in mind, there are alot of possible negatives to your roll as well.
If you're playing a Clan character with a phenotype, your target number would be 1 lower due to the Field Aptitude bonus (assuming you're using a gunnery that is used by your phenotype).
What guardiandashi is talking about is if you're trying to translate your skill in the RPG to the standard board game.