Assuming these sentences are connected and you're talking about the rear view camera, why do you need to guess? It projects two white lines on to the image.
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Line the end of those closest to the bottom of the picture up with an object in the real world. Exit your vehicle and observe how close said object is to back of car (assuming it's the same as my Y it'll be basically the perfect safe distance for an obstacle to be behind the vehicle). Return to car enlightened and able to reverse with confidence.