The author tested several key thresholds:
💎95-100: SS tier, can upgrade a class growth of S to SS.
💎85-94: S tier, can upgrade class growth of A to S, except for Strategy.
💎70-84: A tier, can upgrade class growth of B to A.
💎50-69: B tier, can upgrade class growth of C to B. So far, only the War Elephant's strategy growth starts at C. No other C tier examples have been found, possibly because no other attributes are below 50.
💡Conclusion:
A character with a single attribute above 95 can upgrade the corresponding primary class growth from S to SS.
A character with a single attribute above 85 can upgrade the corresponding primary class growth from A to S.