"About 20 minutes of daily sunshine on 40% of exposed skin produces ample vitamin D in general, but such estimates are gross generalities."

That's an understatement. You comment on the effects of skin color but don't even mention season or latitude. A person sunbathing in Miami, Florida on June 21 is going to get a LOT more vitamin D produced than a person hardy enough to stand naked for 20 minutes in Edmonton, Alberta (or even Seattle, Washington.

Supplementation during winter, especially at latitudes remote from the equator, is essential.