When you get a chance, go out into some open, flat water and try maneuvering astern. You'll get a good idea of how much throttle you need before you have control of the boat.
When I'm backing from my slip, I have learned to not force te boat to go where I want it to but to let wind, current and prop walk do it's thing an adjust my maneuver to that. Fighting the boat without enough water over the rudder is futile.
When going astern you can also turn yourself around and face aft, that way you'll get a better idea of how much control you do or don't have. Just check the bow every now and then to make sure all is clear. And NEVER let go of the wheel when moving astern.
Search this site. There's tons of info and you'll probably learn more about sailing and your boat than in any class. There are many people here who give sound advice and direction.... You'll eventually see who they are!