To me the definition of an ethical shot is:

The soft route to the boiler room, that I know I can hit. If there is any doubt that it can be done, I don't take the shot.

This confidenceCAN increase with time. It is not experience alone though that will increase this confidence. I know some seasoned hunters who still choke at the moment, and some beginners who will slam it home every time.

It all boils down to the individual, and his own personal capabilities and knowledge of what he or she is up against.