[Fantomen]

Precision is used on the battlefield for ranged attacks, both ranged weapons and spells. It is not used in rituals.

Penetration increases the chance to overome magic resistance, consequently it is only used in spells that are resistable with magic resistance. Leprosy is described as resitable mr easy, but I'm not 100% sure how it works for mass target rituals. That would be a good idea to test.

Edit: after some IRC discussion, the opinion seems to be that penetration items work for spells like Leprosy.