Yes I think that is the best solution

Other ways would be to make another format to use for higher levels and connect to "Heading 6" (which is not used yet)
But I foresee troubles.

Third possible solution is to instruct the user to manually indent "long chapter headings above 10"

Also this solution is bound for trouble.

Fourth: Make one template for big documents with long headings and another for not so big docs.

I will vote for the first