Well I guess we need these some facepalm moments to learn healing better !

Finally, I would say knowing the fight a lot really helps to heal correctly (above all to manage MP for the WHM). This prevent you from using a benediction (my favorite example, I know ) on your very low dps if you know you can just Cure him because no incoming damage will happen instead of loosing your CD because you get panicked. Same for any others situations, if you know the fight, you won't use the same spells at specific moments than you would have the 10th first times. In fact, knowing the fight more and more completly change your approach. You'll know when you can DPS, when you can just doing nothing for 3 s, when you can precast some spells like SS or Medica II, ect.