I have two 10% chance to heal astrals: Refined blessed health and Enhanced regeneration. Both are currently level 3 as I wanted to test them out.
L3 RBH gives 10% chance to gain 1200HP when attacking
L3 ER gives 10% chance to gain 30% of damage dealt as HP
My question is Later on, will the % damage be better than the flat rate HP gain?
Short answer, yes.
Long answer, your damage done will most likely grow at more than a flat rate at higher levels, especially in PvE. Also, the % is better in the long run because you can increase the heal in two ways: level the astral or increase your attack power (and more attack power is useful to you in other ways). Whereas for the flat rate, there’s only one way: you’d be forced to level the astral and your damage doesn’t matter.
At lower levels, the flat rate would be much better, but once your damage starts getting big, go with the %. I’d keep both and switch out later.