I would recommend you read the description of specific skills since all of them have slightly different mechanics. Conjured Ward and Annulment (and their morphs) scale from your max magicka but their maximum size is capped by your health (at 40% or 50%). Bone Shield and most class shields are scaled directly with your health. Some shields like Psijic passive do not scale at all, they are always a flat number.
Conjured ward and its morphs (sorcerer class shield), and annulment and its morphs (light armor shield) scale by max magicka, but have a cap by health. Other shields have different scaling, so it's easier to be more accurate if you ask about a specific damage shield.
And here i was thinking this was about necrotic orb+ morphs ...
Would help a lot if you were specific in your questions, since damage shields use a variety of different scaling mechanisms as has been pointed out above.
I was speaking about all damage shields per ability or gear proc. IE Psijic passive, Frost staff heavy attack shield, undaunted ability, Etc... sorry, i should have been more specific. How does the Health cap work? do certain shields cap at a % of health or my actual health total?