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How does one calculate efficiency? Also, how did we find out that Healing charges contribute to defense stat in the first place?

...cast heal.. see defense go up... use magic to find out that defense went up and that defense came from healing charges...

Mini and maximum efficiency o-o not sure about rest.

As kevin said, its not hidden. You can see your defense go up in the character window when heal is charged.

Oh okay. What about efficiency?

[1]
At r2, each charge of heal heals 33 hp. 33*5=165 total health restored. It takes 32 MP to make those five charges. 165/32=5.15625. Round that up to the nearest tenth and that's 5.2 hp per every 1 mp. Of course, this isn't so accurate anymore because it's now a range instead of a whole number.