To solve this problem mathematically instead of clinically we can make the approximate assumption that the human body is slightly less dense that water because it floats. Let's assume the density is 0.90 g/cc which is the same as 0.90kg/l because 1cc=1ml. Weight/volume=density=0.90. Therefore 73/volume=0.90 and volume=73/0.9=81l approx=81000cc. The height in metres is 1.7m approx=170cm (1"=2.54cm).
Volume=area x height, so area in sq cm is volume/height=81000/170=476.5 sq cm=0.04765 sq m. If the dose is 125mg/sq m, then the daily dose is 125 x 0.04765 mg = 5.96 mg which is 6mg when rounded up.