The various tenacity levels all include being damaged as a trigger event, from 50 to 25 per cent damage. (Or, in the case of cowardly, any damage; and in the case of brave, ‘suffer enough for ToP check’). I cannot, however, see it specified whether this is supposed to be n per cent of total hit points or n per cent of current hit points.
I have always assumed the latter to be correct; here’s why: Let’s say the party meets a greater crocodile with 100 hit points. Its tenacity is usually brave, but I’ll up it to fearless, because its nest is just nearby, demanding a tenacity check whenever it receives ‘50%+ damage in one hit’ (HoB p. 12). At second one, it suffers a 48-point hit to its back in one huge blow, reducing its hit points after applying DR by 40 % to a total of 60. This is less than 50 % damage, so no tenacity check is required. The next hit is just as powerful, in fact to the number, inflicting another 40 points of damage. However, this time around it is already severely hurt, so the same numerical damage proportionally does greater damage; 40 points damage out of 60 points remaining is far more devastating, since it actually reduces its health by ⅔. It checks for tenacity, fails, and flees back to its nest to protect its hatchlings.
Am I reading the rules as intended (it is not in fact ‘as written’), i.e. should tenacity checks be done by current health?