Yep, I think that's the way to go, it seems fair for everyone.
And, even if that's not necesseraly my favorite aspect of the game, one of the great things about the current Ars Magica line, is that you can indeed find a ruleset that will fit any mystical being that you can imagine. There are actually so many of them, that I tend now to use only the simpler ones/the ones I know better, and even like that I always find something that works for all the NPCs I can come up with.