One way to do it could be to gives cores instead of claims on provinces in your culture group. E.g. When forming Russia you get cores on all the provinces in the "steppes" and "russian region" which have Russian, Byelorussian, or Ruthenian culture. You only get claims on provinces of other cultures. Qing would get cores on most of China because Manchu is in the East Asian group. One exception should probably be Germany though.