I usually have a systemic approach when placing players into categories. Starting punters, kickers, punt returners, and kick returners go into major contributors. For starters, my cutoff is that they have to have started at least a half a season. Perhaps I should increase this number to a full season, but for now I think it suffices.
I disagree with zoogies, your breakdown looks pretty fair and well-researched to me. It'll be interesting to see how this analysis looks mid-season and post-season.