I think thoses depends of the base piety of your race, as the MP costs are flat but the MP pool can slightly vary.

Thus, the exact % of cost will slightly vary, and raw values might be more or less usefull depending of state.

Are you sure of your calculation between siphon out and inside of grit, plus dark arts/carve and spit cost and gains ? They are meant to be the double of each other, but 12.56 doubled is 25.12 and not 25.32.