[Twan]

If using more than one gem was really intended (KO is the the guy who designed the mechanic, when Bruce Geryk in the manual just described how he understood the game was supposed to work, so I'm not sure the manual has to be > to KO view on the subject)

Anyway, a good formula for me would be something like :
(carried number of gems) / X rounded down (X being a fixed number between 5 and 10)

It would allow the player to be sure that mages having less than X gems don't use them just to reduce fatigue, but also to make them use several gems by giving them a lot.

A good value for X would be 9 because 8 gems is the maximum gem cost for a battle spell (so with X=9, giving 8 gems to a mage and using a 0 gem spell in round one, you can be sure he will still have 8 gems to cast even master enslave in round 2 instead of using one to reduce fatigue on first spell).

Then if using a formula is considered too complex, I'd simply say one per spell.