So in the end, the spell didn't help you survive anything, because even though you survived that 120 damage, when the duration ends, you will still drop dead with -20 HP.
The spell serves no purpose as it is in it's current state. The only POSSIBLE use that it can have is allowing you survive one shot kills from super power attacks, and even then, this will only happen if you manage to heal yourself a sufficient amount before the duration wears off. This makes the spell even more of a hassle because now you can't safely use it without coupling it with a suitable restore health effect.
I hope I can make you understand. This effect is nigh useless, and quite desperately requires this change.
No, it's not useless. It gives you buffer health for a short time, but doesn't let you go getting sliced and diced. I actually greatly prefer the current way over any of your suggestions; it seems silly that it would magically turn into a percentage of your health. It seems much more game-balancing, logical and fun to have to heal yourself before the spell wears off and not make it some kind of quick-tank spell. That would make it useless.
Considering that health is a numerical representation of how much blood/limbs/skin you've lost compared to how much you should have, it makes a whole lot of sense that the spell gives you a bit more, but it doesn't stick around forever. You can take more damage while the spell is active, but once it wears off, your body doesn't have the magic supporting it and can only handle a limited amount of damage. If you took more damage than you can usually handle, you die. Similar to an adrenalin rush where the consequences don't catch up to you right away but you're going to feel it later. I think there's even precedent for that sort of fortify health in RPGs.
So, obviously, I voted that it should be left out. Sure, it's not a problem if it's optional, I just won't use it, but I think it's a waste of time implementing. It doesn't make sense, where the existing system does and offers a bit of a challenge, makes you think before you use the spell. Elaura's version makes more sense than this percentage thingy, but I'm not sure which I'd prefer. For the moment, my vote goes to "don't go breaking things"/no.