How to retrieve effective damage value from the "before entity is hurt" trigger

I'm trying to make my own totem of undying kind of item. The "entity attacked" and "before entity is hurt" procedures both provide a dependency called "amount" which represents the raw damage value dealt to the dependent entity, but I need the damage value which is calculated after considerations such as armor, armor toughness, armor enchantments, potion effects, etc. Am I going to have to calculate the effective damage myself or is there something I'm missing?