Carbuncle has the job trait Magic Defense Bonus IV (MDB+16) at 70 or 75. His Auto-Regen is still 1 hp/tick at 90. No form of MDB V I believe or not before the level 86 at least.

Avatar's hp at 90 (thanks to Tarsal Slam and Banneret Charge)

Celestial avatars : 1420 HP
Carbuncle : 1359 HP
Fenrir and Diabolos : 1420 HP
Spirits : 1277 HP

Their Healing Blood Pact are based on a % of their own max HP, easy to check with HP atmas or inside Shantottofication BC.