I don't have exact data, but I did some simple testing in offline raids.

Aiming for the head, Tagilla once took 10 flechette darts. One pellet hit the thorax from the rear (damaged to 59/60), and post-raid screen showed about 210 dmg dealt. Next try, I dealt 224 dmg with 13 darts, and damaged armor to 50.5/60) - my guess is, head HP is doubled to 200.

Aiming for center of mass, I dealt him 1853 dmg in total (according to the notoriously inaccurate post-raid stats screen) and killed him with a stomach shot. I didn't have much luck with shooting and killing only him.

Overall, my guess is his HP is doubled, though individual body parts may have different modifiers.