@ David. Agree. But definitely by not one full stop. That (1 full stop) would be the case if the clouds had only forward scattering; then they would distribute on the ground a flux equal to the incoming solar flux. But as we know from air travel, clouds also reflect a lot of sunlight towards deep space. Which can be confirmed without flying: under a cloud cover the light flux is less than under direct sun. Closer to 1/4 (2 stops from sunny 16). So, in the most favorable case (under a small clear hole in a sky otherwise covered with nice white cumulus) one might hope for 1+1/4=1.25=+1/3stop; rough, quick estimate.Agree or refute: I do not bite. - David Lyga
@ other readers. If you don't agree, I won't go into one of those sterile forum controversies; just use your light meter under comparable conditions (same period of year + same daytime = same sun elevation).
Measurement worth thousand theories. Confucius. (or, was it Galileo? or Groucho?)