PvE resist mechanics, dmg intake
 

I'm having troubles finding a reliable consolidation of up-to-date resist mechanics. Can anyone refer me to a site with good info? Or if not, then can anyone give a general rundown?
specific questions concern dmg intake from PvE mobs:
how much player resist provides how much % avg mitigation?
how does resist scale on mobs high/lower level than the player?
are there tiers of resist effectiveness? i.e. do you gain more avg mitigation with 25% over 20% than 30% over 25%?