Starting there, you need to open up about two stops. As always, if you are in doubt and have the time, bracket your exposures. Focus on the eyeball With a long lens, you don't have a lot of depth of field, so use it wisely. An animal's eye is the most important part and it should be razor sharp. A highlight also improves the photo immensely, and gives you something to focus on.
Some animals have jet black eyes which look like holes in their heads without that highlight.
The tolerance distance for each animal is different, and you can't predict exactly what that will be. Sometimes the animal may accept or completely ignore you. Others will walk or run away, which only leaves you with a fleeting butt shot. A bird that feels threatened won't come to its nest, so you won't get any shots. If you have an uncooperative subject, it is pointless to harass it, so please don't.