How is the bonus HP calculated?

Does it take a % of your HP every level and add the bonus from that?

Or, does it calculate the % every time you level and after all the HP from items is added?

If it were the latter, I feel like the Volibear/Warmog nerf is going to be balanced out by the HP you can get from these new runes/quints.

It comes into play after your max HP and items combined.

So considering the Volibear nerf:

Two Volibears have the same item build and end up with 4000 HP by endgame. One is post nerf with full HP% Runes and the other is pre nerf when Frenzy did more damage.

Pre Nerf:
Frenzy does 20% Bonus HP as damage and 290 base damage.

Volibear with 4000 HP
Frenzy does 290 + 800(From HP) + % of Targets missing health = ~1090 Damage

Post Nerf:
Frenzy does 18% Bonus HP as damage and 260 base damage

Volibear with 4000 HP (Plus 10% from the Runes/Quints)= Increased to 4400 HP
Frenzy does 260 + 792 (From HP) + % of tagets missing health= ~1052 Damage

Prenerf: 1090
Postnerf: 1052

They shaved 38 points of damage off the skill they nerfed, with the addition of the new runes.

Lol, good thing I love playing Volibear.