[Squirrelloid]

Ok, first of all, there's absolutely no way to predict what the AI will have spent. Because the AI does not behave rationally.

Second, in this particular case, the appropriate strategy would have been to cast Fate of Oedipus, which not only automatically cancels Eyes of God, but also blinds the caster. Then you could cast your own =P.

Third, it is generally a better idea to overcast than to dispel and then cast. Well, its actually a better idea to kill their caster, but barring that...

Fourth, how overcasting and dispelling is calculated.
(1) *Naturally* having more of the path required to cast it than necessary gains you a benefit of +5 per extra path length to the check. So if an N7 caster was overcasting Gift of Health they'd get a +10 to teh check. Boosters do not count for this (but empowering does).
(2) Every excess gem you spend adds +1 to the check. (Note: if you have a site which decreases the cost of the appropriate path for the global, you will only benefit from gems spent in excess of its *normal* cost, not the cost with the site factored in. Ie, the excess you spend to make up the difference between the reduced cost and its normal cost is lost with no effect.)
(3) Both sides roll a drn and add their bonuses. Higher total wins, tie to the defender.