I didn't forget to account for anything. 20%+lostHP is the exact same as max hp+20% as far as taking damage is concerned.
Assuming +Wyvern HP swaps will work, here is a dumbed down example:
a 1k HP wyvern gains 200stoneskin, effectively giving it 1200HP.
If that wyvern was beaten for 500dmg, he would have 500hp, and would gain 200 from the 20% of max, and an additional 500 stoneskin, effectively giving him 1200HP.
Same thing if you swapped in say, 200 more HP.
During the abilities triggering, a maxhp'd wyvern would have 1000/1200HP and would gain 240HP(20%) +200 from missing HP, effectively giving him 1440HP, even though his regular max is 1000.
Same thing if he was damaged 500HP, except he would gain 700 extra stoneskin instead of the 200 effectively giving him 1440HP again.
Realistically, with Wyvern mail, chanoix gorget, drachen brais+1, homam gambieras, lancer's torque and earring, you should be able to trick the game into giving your fully healed wyvern 1266HP and 602 stoneskin, effectively giving it 1828hp.
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