I suggest the following:
- Each point of Charm should give a 0.1% bonus experience.
- Each point of Charm should give a 0.05% chance for an additional drop.
With this system
A character with 10 charm would get 1% bonus experience and would have 0.5% chance to receive an extra drop.
A character with 100 charm would get 10% bonus experience and would have 5% chance to receive an extra drop.
A character with 400 charm would get 40% bonus experience and would have 20% chance to receive an extra drop.
A character with 800 charm would get 80% bonus experience and would have 40% chance to receive an extra drop.
With this change charm could become an useful stat and would not affect class balance in any way.