Finding the exact numbers the game uses could be tough, though I definitely think it can be done.
The new bar for shield health has a dynamic length varying with the max health of your shield. If we go simplistic and assume 1 pixel on the bar = 1 damage, we can declare an enemy to deal 100 damage if 100 pixels go. It doesn't matter all that much if the actual damage is 200, because we'll have a lot of numbers we can compare relative to another. We could also later compare these numbers to some studies on damage to our actual health and end up with a very good idea.