The secret to teaching a dog to back up straight is... reward placement. Throw the food to where the dog is supposed to back up to - where they happen to be is irrelevant. Check the angle of your feet - that sets the line of travel and also determines where you will throw the reward - food or toy. Check where you are looking - you should look where you want the dog to be, regardless of where the dog decides to go. Now - first teach your dog to back up a few steps, however you want to do that. When that is solid, add in the idea of "Straight" And straight is a function of reward placement, the orientation of your body and your gaze. Check out this video.  My feet face the cone.  I want my dog to back towards the cone.  When he veers I ignore that behavior and toss the treat towards the cone -where he is supposed to be. He gets better!  With time, he'll get better still. Give it a shot.  If you throw TO your dog, you're reinforcing their choice to go wherever their feet take them.  Give them a reason to back up straight - the reinforcer goes straight.