Ok, how about base 14, and requires four ranks in Bluff and Diplomacy, and two ranks in Preform? Keeps it reasonably accessible but will make it a bit harder for Cha heavy Teir 2 and higher classes like Sorcerer and Favored soul to tack on in order to load up on extra castings a day/Higher DC's/Other goodness form a Jacked up Cha?

Seem reasonable?