Backing a flat bottom boat can be tricky. They tend to resond to thrust only in general direction, and often with no kind of orientation. Wind and current will have a huge effect; you sometimes will find yourself going in the direction you want, but sideways.
What I used to do, was to use quick bursts of power to get the boat oriented the way I wanted: For example, if I was backing up to the south, and the boat started pointing (not drifting) more to the west as it traveled, you'd turn the motor to the west (or southwest, at least as much as possible)and give it a sharp bit of thrust to bring the nose back in line. It does tend to pull you off line, but it keeps the boat pointing the way you want. Usefulness depends, of course, on how much room you have to work with.
Hope this helps;