Most of the "Resist Magic" abilities listed in the book are pretty specific as to whether they resist Godplane, Soceryplane or Spiritplane powers. ("Resist Pagan God", "Fight Spirit" and the like).
For a generic "Resist Magic" Ability, I'd probably treat it like a "Broad Skill" and charge extra HP for advancing, and require improv. penalties on all uses of it. (better explained in HeroQuest).
RR
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