Halichos Bas Yisrael 4:4 (note 6) cites Mishnah Berurah 75:2 as saying this, although I haven't found it there (he just says that "her face and hands, whatever is normally exposed according to local custom," are not considered ervah as far as a man saying Shema). HBY also references Kuntres Malbushey Nashim, but I don't have that to see what he says.
One possibility: we find that halachah considers the neck area (in an animal) to extend downwards until the upper lobes of the lungs (Shulchan Aruch, Yoreh De'ah 20:1). Since in a person this point is demarcated by the collarbone (see image here), that may be a reason to use that as the dividing line between the neck (about which there's no statutory source requiring it to be covered) and the body.
Edit - looks like my facts in the second paragraph above are wrong - normal human lungs apparently extend some distance beyond the collarbone (as in this image of a chest x-ray). Shulchan Aruch there does say that the dividing line (for the laws of shechitah) is how far the lungs extend "when the animal stretches out its neck to graze," so maybe for tznius too, the same principle would operate - we'd have to know where the upper lobes of the lungs are when a person extends his or her neck. I guess we'd need to ask an M.D.