The average damage is simply the average of 4d8 + 20, as you identified. So 4 times 4.5 (the average damage of a d8), plus 20. 38.I think understand your math, and your formula appears to work; but, in your formula, where are you getting the idea of taking the average amount of damage of a given attack and plugging it into the formula? Is it some sort of implicit math-thing?*note: I suck at math* Maybe you could make an actual formula with notations, for me, for these kind of attack powers for monsters?
