It is in the game guide. Your skill in intent determine what spells you can cast. A power level 4 harm/fire spell requires harm at level 4 to cast. Base determines how much the casting cost you. Essentially, you divide the power level by the base to figure the cost. So a power level 4 harm/fire spell costs 4 to cast if your fire skill is at 1, 2 to cast if your fire skill is at 2 and so on. That is how I recall it at least...
