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.
I've got to be honest, I'm not sure where the best place to start with this would be. Omega Shell, Ancient Plate and Volcanic Plate would be very good for getting raw damage numbers on projectiles. Comparing Volcanic Plate to Ironmight Plate in FSC would let us find out exactly how much status defence on shields matter. Dragon Scale Shield would let us find out raw damage numbers from shadow enemies that are neither poison or fire. We should also compare status enemies to non-status enemies.
After figuring out some raw numbers, we can do things like comparing Skelly Shield and Crest of Almire to find out how much defence on the shield matters. UVs will let us fine-tune the results. The important thing is that we have a meter that can be measure in pixels, which is a very finely grained unit of measurement.
What you need is a stationary enemy which can be found alone, which deals damage regularly. For testing against status, I'd suggest the status grates in the ATH and maybe the Gran Faust charge for curse, with no curse defense or weakness.