Perhaps its been suggested already but before I had a camera I used a method that worked for me. I backed up to something close, got out and visualized the position and the offsets needed to correct it. I put a penny on the ground next to the drivers door and left the door open and watched the penny. The left/rignt corrections took some learning but the backing distance difference is easy to judge.
No man has a good enough memory to be a successful liar. Abraham Lincoln
2016 36FBTS Pinnacle
2016 F350, 6.7, 4x4, DRW, long bed
B & W Companion 5th wheel Hitch