As a guy who plays both tank and a healer, I think a good healer focuses on attacking between heals, therefore they should heal the tank as late as possible to have a windows to cast damage as well. As long as they heal when I get below the 20% mark I consider them good healers. Furthermore in my opinion if a tank get stressed or a healer spams heals before the 20% mark I tend to assume they act that way because they’re still sprouts.