[Nick_K]

The maximum number of gems used 'N' includes the gem to raise level, gems to reduce fatigue *and* gems needed to cast the spell. An astral 1 mage can never cast a level 2 spell that also requires gems to cast.

An astral 2 mage that casts returning can not use any more gems to increase level or reduce fatigue, because he's already used the 2 needed to cast the spell and he cannot use more gems than his level.

I thought that in dom2 at least higher than 200 fatigue resulted in damage and could kill mages - a common cause of death for communicants. I though that the high-fatigue battlefield spells essentially required a very high-level mage or lots of gems to reduce the total fatigue below 200. Of course, I could be wrong here.
I'm pretty sure that the spell AI will refuse to cast spells if they'll take fatigue too high.