[RonD]

My experience is that AI will usually go ahead and use the maximum number of allowed gems, at least for the big-fatigue spells.

One thing I am more confident in (from running a bunch of tests in which I was teleporting an S9 Oracle + several Grand Thaumaturgs to cast Master Enslave) is that the AI is predictable - it will always use the same number of gems for the same spell given the current caster level. So running a test should let you know how many extra gems you need to carry in order to get a second spell (Returning, or Vortex of Returning) to cast.

As to why your tests varied in whether or not the Oracle cast Returning - I recently learned the very hard way that Returning is not automatic. The AI sizes up the opposition (a good question is "based on what?" I *think* it is based on the ratio of your total HP to the opposition's total HP). I would guess that the times when you Oracle did not cast Returning were cases where the Master Enslave was more successful, tipping the total HP balance more in your favor.