Blocking/parrying eats away your stamina, right?
You must have stamina to block/parry, right?
If you do not have enough stamina to even raise your shield, taking a hit should be devastating one, right?

In my view, staggering should not happen according to health, it should happen if the hit depletes your stamina pool.
What mechanics high health VS near death should have? Should health boost up stamina?