I personally generally start hard-casting heals at the 50% HP mark unless I have CDs up - Not because I need to, but precisely because I know tanks tend to panic when their HP gets much lower. If I let them drop much lower, they commonly start using all their CDs at once >_> But casting any earlier could be overheal if I crit.

When I tank myself, I simply got used to having wonky health levels. I vividly remember doing huge no-grit pulls before Stormblood and my health was yoyo-ing all over the place thanks to DA-abyssal drain spam and taking massive damage at the same time. "60% - 90% - 40% - 95% - 35% - 95%". The fun times before the double blood price nerf.