I was wrong about WHMs.

[Tranquil]

-snip-

Truly excellent post right here, very true stuff. What you mentioned about "snap decisions" is one of the biggest factors in all healing game in general, at the top level. That kind of adaptibility is very important for max synergy and minmaxing mana usage, every GCD and so forth.

Overhealing never kills anyone (literally), but some times there might be some unnecessary MP strain due to the thing you mentioned: Ie. overkilling a heal after tank killing move. For instance, for a T12's Revelation, if there's a Virus, E4E proc, Rampart, Stoneskin and a fresh Adloquium up - there might or might not be a need to queue up that Cure II then, depending on whether or not you also camp your co-healer's casting bar and if you can see they are pre-casting Physick/Adloquium to land post-impact. If they are not, it might be a safe bet to land that Cure II - stuff like that. Some of it is also RNG of course, with Blocks happening or not happening, even with CDs on. Adapting mid-combat requires good awareness and snap decisions so it might not always be so easy to not default into that Cure II. All of this adds up and it sounds like that 0.3% you mentioned could have come from experience, first hand.