Charm has been coded and recoded many times over the course of the server; when I joined charisma had a huge, huge effect but I think now its quite minimal. Personally I go for over 200 cha just because its easy with enchanter-specific gear and also for the incredibly powerful Lull line. But I gear Loraen for HP/AC above everything else.
http://wiki.project1999.org/Magelo_Blue:Loraen
Oh and also I once did a test of 20 charms (255 vs 200 cha) and rapidly realized that the variance on each individual charm was so large that it would take hundreds of samples to get an accurate formula.