If i remember right. at Vr14 32,000spell resistance reduces all incoming magic dmg by 50% anything lower than 32000 is scale down to its respective percentage, and anything higher adds nothing. It counts for all types of magic but increasing any specific resistance adds of course only to that element. And if you are blocking a magic spell when you have 32,000 resist you reduce the incoming magic dmg by 75% there are of course other ways and things to reduce the dmg taken but that is the gist.

Here are things that can add to dmg mitigation:

Major Protection: 30% (Veil of blade)
Minor Protection: 8% (Ring of preservation)
Major Maim: 30% (Nova)
Minor Maim: 15% (Deep slash)

Blocking: 50%
Capped resistance: 50%
DK passive: 10%
Nord passive: 6%
Sword and board passive: 20%
Deflect bolt passive(only if projectile): 15%
Footman set: 12%
Absorb magic/defensive stance: 8%

I might have missed some but do note that these give diminishing returns. the math is like this you take the % thats gonna be left after you remove it but in decimal form so blocking is 0.5 and say the absorb magic is 0.92, what you do then is you multiply them. So 0.5*0.92=0.46 which means that you reduced the incoming dmg by 54%. The max I have been able to reduce icoming dmg on my DK Tank was 89.5% and that was with nearly full armor, footman, deepslash, absorb magic and ring of preservation. the highest hypothecial is ~95% but you would need both nova, veil of blade and all the other skills for it and it would only last you around 10sec