I agree that making the playoffs are likely one of the main factors for ranking. I believe that the 8 teams who make the playoffs all get the same credit and league finish is secondary, even among those squads. The teams that make the finals get another boost, and the champion gets yet another boost over that. I could be mistaken but thats the feeling I've gotten.
As far as your squad being the 5th best team thats very hard to say, but you're definitely in the mix of teams vying for that in my opinion. Unfortunately the rankings can only be based on results rather than club strength, which varies quickly with training, acquisitions, and game shape.
As for you shooting for the title in season 13, I personally love it when people set high goals (as they should!) because it brings the level of competition up. Higher levels of competition = more fun!! Just so long as you understand I aim to do my best to stop you! That said, best of luck!