The secret to teaching a dog to back up straight is…
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.