What amount of int would the charm need to have higher damage casting from acc slot?

In short, never.
The only time where it's of any use is when you're offcasting in a group climb and are trying to preserve mana. Mages/Warlocks/Alchemists in whistle climbs should generally have enough mana after a bit of help from gear at level 1-20 to not need an int charm.
For solo'ing i'd recommend just using the shrine.


This is why int charms in general dont sell, the very best ones get some interest, but not a lot