Hmm.. I am sorry, but..
effective resistance value = Rb + max((Lt - Lc) * 5, 0) - min(P, Rb)
Rb - target base resistance (as advertised on the character sheet)
Lt - target level
Lc - caster level
P - caster spell penetration
so for usual boss (not Winterchill/Supermus): effective resistance value = 0+ max((73 - 70) * 5, 0) - min(xxx, 0) = 3*5= 15
Damage reduction percentage = (effective resistance value / (caster level * 5)) * 75 = 15 / (70*5) * 75 ~= 3.21%
and I checked many times - damage loss much closer to 3.21% then to "5-6%"..
used:
Resistance - WoWWiki - Your guide to the World of Warcraft