How does affinity work exactly?

[fateriddle]

My basic understanding is that 50% fire affinity means healing 50% of the fire damage dealt.
My questions are:
1. Does healing base on original damage, or final damage after all the shield/resistance calculation? And if you take no damage(bone shield) from a fire spell, does it still heal you?
2. If that fire spell would kill you, is the healing done before or after that damage?

[HousePet]

Based on original damage and done after.