Suppose your scope could calculate and display your hit likelihood in the vitals of a deer based on distance and the accuracy of your gun/ammo system.
What would you consider to be a minimum hit likelihood?
100% (always make the shot)
90% (9 out of 10 hits)
75% (3 out of 4)
67% (2 out of 3)
50% (1 out 2)
IMHO "ethical" requires 100%. Taking a shot with anything but 100% certainty means some other factor is outweighing ethics. That may be necessary in some situations, ethical or not.
As Jordan already pointed out, even if you have 100% confidence in your ability to place the shot, 100% in the outcome is not possible.
And I know some people like to say anything less than 100% is "unethical" but think about it for a minute, you're shooting at the animal and trying to kill it. It's not painless for the animal. People talk about "a humane kill," and sometimes you get the pinnacle of that with a CNS shot that turns the lights out instantly. Sometimes you don't, and the animal might run a hundred yards and take a few minutes to expire. Is one an "ethical" and "humane" death and the other not?
Some people would say so.
