Aaron2 said:The spell also has the silly feature that your own Charisma actually makes it harder for you to save against it. I can see a bard taking off his Cloak of Charisma before casting it.
It needs a fixed DC to work the modifiers off, which as you say should NOT be the standard formula (10+Level+CHA mod) - I'd suggest DC 20 as the base, maybe DC 25 given the favourable mods.