This cap, I believe, is a direct ratio of your total int-based mana. Someone told me that SEQ showed a 4164 total mana pool unbuffed, even when the pool should be higher. Since 4164 divided evenly into 3, AND the resulting divisor was '1388' I concluded that not only was the 4164 figure accurate, but the 'cap' was actually a function of your total mana pool, basically, 50% of your mana pool is the cap on +mana items. You say this 'limit' was known for a long time, well, not by the general public. I knew there was a cap on +mana under level 20 or so, but I had never heard of any limitation above lvl 20.