samclauw great question. With AAM you have to be quite explicit about what content you are restricting. For any plugin, it would be near to impossible to determine approximate 10% of "allowed" content. I can list several possible scenarios where it just not possible. For example, let's say you want to show the last 10% of the articles that were published. In this case, when tomorrow you'll publish a new article, visitor can read it as it will be in last 10% and overtime, visitor can read 100% of articles.
My best recommendation is to be more explicit about what exactly you want to show to your visitors and what is allowed for an authenticated user.
You can create a tag "Visible Content" and tag any post/article that you want to show to your visitors while all other posts/articles TEASER MESSAGE to read unless a user is authenticated.
I can help you to setup this all if that is something that satisfies your requirements.