I don't think Stats are part of the calculation since I tryed DRK/RUN with a Great Sword with no Stats like STR and INT fully naked then put on my Twilight Mail Set and saw no change in the En Spell damage. I think the Enspell damage effect from runes is purely calculated from weapon. Though it might be effected by things like Enspell damage + that comes from the earrings Lycopodium Earring and Hollow Earring though the only way to find out is to find someone who has these earrings. Kind of sad though that other then these Earrings everything else that has this stat is a Sword that is RDM only.