[Alneyan]

Gem use should be limited to your level, so you might need a level 8 mage to cast the spell in the first place (I'm not sure if 7+1 would work). It also means a level 8 mage should not be able to lower the fatigue cost by spending more gems, as he has already reached the limit of gems to be used (8 to cast the spell).

In any event, fatigue higher than 200 doesn't matter, so don't give your mage extra gems after that: 800 will go down to 200 on its own. You should only use extra gems if that will bring your fatigue low enough to cast another spell; *very* high level Astral mages will be needed to pull that one (or a Communion).