I'm not sure that's the case. It looks like you can set your linebackers to either line or cover, and any linebacker or DB can blitz.
So what is the difference between the blitz percentage in the playbook, and the blitz percentage in the formations? If I set the blitz percentage on a playbook for 30%, then set all of my LB's and DB's to 100%, does that mean that I would blitz 30% of the time, but when I do I send everyone?