P. 16 §6.4.4 of the manual. The penetration value is (11+((magic skill of mage in relevant path-magical skill level needed for spell)/2)+2d6oe). Compare to resistance value: (mr+((the targets skill in relevant path(the path of the spell being resisted))/2)+2d6oe). Pen-value > res-value = ouch. For spells affecting the entire battlefield, use 7 instead of 11 for pen value.
I just got the game, so I read this yesterday
