Nostalrius Begins - Quality wow vanilla realm (1.12)

Hi, anyone knows the magical resistance formula to know the raw % chance of resisting a spell in PVP?

Like, if i have 150 shadow resistance, whats the chance of resisting shadow word pain?

Read up about Spell Penetration (and thereby resistance) here:
http://vanillagaming.org/forum/index.php?topic=15951.0

The formula is 15 * Boss Resistance / 60 = % of damage lost