Is there an exact formula? It looks like military units is part of it, but, not necessarily by their "power" but just by having X amount of units?
I also notice even though I don't lose any units it goes up and down per turn which is strange. It doesn't change a whole lot but it does go up and down about 4pts per turn sometimes. I also notice that the AI's I have set just a difficulty above challenging (hard mode I think it were) have 2x and 3x more of it than I do and they both have started on the spell of mastery as well just on the "hard mode" setting. wow!