General rule is as below:
ATG (11) - can be trained without cap until all his skills are equal to 15
HOF (10) - can be trained without cap until all his skills are equal to 14
MVP (9) - can be trained without cap until all his skills are equal to 13
...
Star (5) - can be trained without cap until all his skills are equal to 9
Potential (N) - can be trained without cap until all his skills are equal to (N+4)
Other combinations of skills You can check here:
http://bbtools.x10.mx/potencjal/