If you're using an add a circuit from the fuse panel under the hood, why not just use a tap and a mini-fuse on the cigarette lighter's hot-wire behind the interior panel? Doing that would probably be less of a PITA, a shorter wire run and a little cleaner of an install. But if you're still going to use an add a circuit, just keep it simple and add it with the cigarette lighter fuse and use the same size fuse.
Is this what you're talking about using?: