I'd consistently pre-cast as a healer (or even as an enchanter using charm when soloing for that inevitable charm break) and then duck out (sit) to cancel the cast if not required or if another more important spell was needed. I believe that if the spell fizzles or gets interuppted by a mob, you should lose mana. I'm not sure if i like the whole once you start casting you lose a percentage of mana based on how long you've been channeling the spell. What do you think?