While the minimum predefined distance is 2 meters, you may define an arm's reach distance which should be at least 500 mm (50 cm or 1.64 f). After measuring the distance between your hand/wrist holding a smart phone and your eyes, you input the distance in "Custom Distance" in Settings, then this distance will appear in the Predefined Distances.
To identify shortsightedness symptoms, the test distance should be at least 2-meter which is longer than arm's reach. Since generally an arm's reach distance is much shorter than 2 meters, this testing mode is good only for casual self-diagnosis for example when you are on a train, and the test result is inaccurate and unreliable. It may be easier to use Self-diagnosis with a Mirror.
Some medical practitioners working in hospital had found that the most convenient distance in hospital beds is around 1.2m, because when a patent is sitting up in bed, this tends to be the end of the bed length, and often because the way the curtains are drawn in the room in hospital it is impossible to get much further back. Therefore, this arm's reach distance could be 1.2m.