It all matches, from top to bottom, of hands and feet, corresponds to, from top to bottom, the chart of the body. Tips of fingers and toes reflex to the head. And heels of feet and hands reflex to the pelvis of body. Everything corresponds in between.
So therefore, the diaphragm muscle separating the bottom of the chest and the top of the abdomen, corresponds to the reflex areas on the hands and feet that are located between the bottom of the chest reflexes ( bottom of big knuckles that are below the fingers and toes) and the top of the abdominal reflexes (softer middle and arch area) of the hands and feet.
It is not a pin point, specific, reflex spot. This reflex goes across the hands and feet just as the diaphragm goes across your body. The diaphragm and solar plexus are so close in the body, and likewise their reflexes overlap part of the same reflex area, making the area all the more important for breathing, relaxation, etc, etc.
Reflexology is a great way to help both the diaphragm and the solar plexus. When I do reflexology on the feet, I most always start working on this special solar plexus and diaphragm reflex area first.