QUOTE(Boggyb @ Oct 15 2010, 22:19)

Spell damage multiplier = 1 + prof / 200
That means a 12% increase to proficiencies equals a 6% increase to damage from elemental spell (which are most of them)
Rather, a 12
point increase in proficiency equals an increase of 6% of
base damage.
Let's say, for the sake of argument, you have 50 Elemental proficiency, and your equipment (without the piece you're interested in) gives you +50 bonus. Your effective proficiency is 100, which gives you a damage multiplier of (1 + 100/200) = 1.5. Adding 12 proficiency brings that up to (1 + 112/200) = 1.56. That increase, as a percentage, is
(0.06 / 1.5) * 100 = 4%
The Battlecaster helmet gives you an effective damage increase of about 3%, though that depends on how much you already have from your staff. Of course, all of this is a huge oversimplification, since it ignores the effect of INT and WIS bonuses on your magic rating, and the effect of armour proficiency on all of the equipment stats...