I had 13,271/21,924 HP (not Zerk) (60.5%) I used HB Which brought me to 13,272/30,000 (44.23%) I did not have an active Zerk. Once I healed & Used Dragon Roar to get to 9268/30,000 (30.8%) I finally activated Zerk. Historically using HB should activate Zerk if you hit the right range. Edit: I just checked at 14k HP / 21,924 > 14,000/30,000 No Zerk until I used a red potion then it activated Zerk.