well if its a warrior, let him drop to 10%, throw bene, drop him to 10% again, he throws equilibrium and IB and maybe a cd or smth. if equi crits he should be on 70% now, if not, hes on 50. now either let him use second wind (if he doenst have it then 2nd IB) + thrill and hes on 100- 110%; or throw tetra, depending on if you prefer leaving cs or not.
if its a pally. well, pallys shouldnt drop below 50 ever. thats what cds are for. also, hail clemency.
and drk. dunno. somewhere inbetween i think?

all in all: tank hp are a ressource like your piety. if you'll never use them, they're wasted. so dont complain when tanks start to lose some hp in favor for str x)