Well it's as far as I see not a 100% damage count, but it definetly increases your magic damage of your magic skills. Try to see the difference with a Melee Damage only weapon and a weapon with magic damage. Then u can see it for yourself, but keep in mind, that it has a gap ofcourse
it's never a fixated number. f.e. a warhammer can have 50-64 magic damage, so it can differ.
