Thanks for your effort! It is just the solution for my problem.I don't know enough about Scion to comment too much but one thing leaps out at me immediately about Knacks. Well two things.
First Defense is way better than an equal amount of Armor, in fact its about twice as good. Each point of armor negates a point of Damage but even a point of extra Defense will negate all the damage a lot of the time. Someone with +4 defense is just flat out not getting hit.
Since there's no Pure Mortals in a Scion game I would think then you might want to make Mortal stunts 2 for 1 Refresh because point for point they are much less potent than Powers and in many cases less potent than just having the extra Fate point.