I usually let Tanks drop to around 30% before I take action, 50% if I don't have insta heals.

I sometimes get the tanks that scream "heals!?", but I really rather not stop in the middle of my DPS just to heal you when I know you'll be fine.

You really gotta trust your healer with this. Just like you know how much damage you can handle, we know how low your HP can drop before we need to heal.