Magic Resistance Formula?

[Nagot Gick Fel]

Originally posted by Zapmeister:
Has anyone figured out the odds of success of a spell cast with penetration P that has found a target of magic resistance R ?

My guess is it's (10 + P + 1D6oe) vs (R + magic bonus + 1D6oe).

1D6oe = 1 six-sided die, open-ended.

P = attacker's penetration, includes bonuses from items + (extra magic skill / 2)

R = defender's MR, includes bonuses from items or spells like Iron Will.

magic bonus: applies if the target's is skilled in the path that's used against it - eg, a nature mage will resist Charm better, an astral mage will resist Soul Slay better. I'd say this bonus is magic skill / 2 (rounded up?), but it's just a guess (hard to test thoroughly because of the open-ended random factor).