My guess, based on some very casual observation, is that the heal effect is increased by the critical damage bonus. So If you've got 100% critical damage and roll a critical on the heal, you heal twice as much.

Hi

You are indeed correct! Apothecary heal acts as any critical hit and scale of the Critical Damage multiplier.

I did some further "research" on this topic and found a thread on reddit, where they say critical damage does NOT work on the healing skill of Apothecary.

On the other hand, this reddit post in one year old, so maybe it changed?

Like I said, very casual observation involved, but I'm using Apothecary myself and have had the feeling that he has healed far more than the 35% (or ~40% since it's fully upgraded) he should. I've always figured that was the critical heal in effect, since he has ~80% crit damage on him.

The thread on reddit I linked here did not say that his critical healing does nothing. But accordning to this thread extra critical damage from gear has no effect, a critical heal will allways get a multiplicative 50% bonus. 

35% (or 40,25% when booked) healing x 1,5. Even 300% critdamage from gear would have no further effect.

GreenKnight said something else, according to his post it "acts as any critical hit and scale of the Critical Damage multiplier". I just want to know wich version is the right one.

That may be the case. If it's 50% or 80% I couldn't say without actually testing it.