As you may now know, "...we'll (Blizzard) be handing out end-of-season PvP awards separately to the top percentages of players in each faction...", the way titles are calculated have changed.
Unfortunately, the leaderboard API has not changed with it and as such it is no longer possible to provide 'accurate' cutoffs as the API only provides rank/rating data (and not faction!)
Due to this change, we are unable to provide accurate faction based titles
Below you can see the ratio derived from the current leaderboards (The top 5,000 players) which can serve as a useful 'guide'
Below you can see the, now redundant, overall titles data. This can be used as a 'guesstimate' for the duelist/rival and challenger cutoffs