Divine aegis can only absorb up to 20% of the casting priest's max health. So if you get enough mastery for a greater heal crit shield to = 20% your health, that's the soft cap.
Assuming it's true: In my current gear, my GHeal crits for about 33k. 20% of my health is on the order of about 20k, maybe a bit more, so about 60% of a GHeal crit. Base DA absorbs 30% of the amount healed; in order to double that to 60%, you'd have to get enough mastery to increase your shields by 100%...which is a massive boatload of mastery. It might be less with better gear assuming a slower scaling of stamina compared to heal amounts.
(Using AA with 5 charges of Eva along with my SP trinket and a few other tricks has been known to give me over 50k GH crits which would produce 15kish DAs, so assuming the 20% of priest's health limitation, the target for being "mastery softcapped" in 346ish gear with AA procs, trinket procs, and other short-term buffs would be on the order of 33%, which would put me near it already...but I don't necessarily think that's a useful "cap" to be concerned with.)
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