From the rules:
You will find that the people who want the most expensive seats will pay their money win or lose (but care a lot about ticket prices), while the less expensive seats are mostly filled by rabid fans who will show up when the team is winning but won't pay to see a losing team.
This makes me think that there is not an "ideal" percentage. During the playoffs, your team was on a winning streak and in the playoffs so the rabid fans were showing up and filling your bleachers. Now it is a new season, so it may take a few wins for fans to get excited and start filling the bleachers again.
The other seats seem more dependent on division level and price so since you are selling them out, I would think you would continue to do so on a consistent basis. Therefore you can probably add more seats and/or raise the prices for those seats.
I don't know what the ideal build would be but those are my thoughts on your situation.