Caster raiding HP's vs. Resists
 

At high level raiding gear choice, is there a sort of formula people use for which how many hp's 1 point of magic resist is worth when it comes to dealing with AoE's?

As an example, is gaining 13 hp's worth the loss of 4 magic resist? etc.

There are very few aoes you should be taking as a caster while raiding really.