OJW, I would like to make a point of correction to keep this thread focussed on the original question. We are aware of and comfortable with the formulae for calculating the amount of a heal, and for the influence getting hit during a heal has.
The question is "what determines whether a heal is successful or not". This presupposes a failure rate for heals that is outside the expected heal rate where the toon has not been hit during the deal delay. This is something I believe I have observed. Thus far no one has provided an answer, opinion or theory to compliment, refute, or refine my working theory.
The point of the question is this: If we can confirm that there is in fact a success check, and we can isolate what the check is based on (I think its the heal skill), this would be helpful information for planning templates that include heal. By way of example: right now the only difference between having 85 and 90 healing skill supported by the available information is that you heal a little more with 90 than with 85. If my hypothesis is correct, an additional difference would be that you would never fail if you were using enhanced bandages, while one in 20 heals would be a failure at 85.
Unless someone comes up with some information that is on point, I will set up some tests.