Let me preface this with the statement that this is not intended halacha lemaaseh.
In this segment, on page 329, Rabbi Falk gives two reasons for covering the leg. (He applies similar reasoning to feet.) In terms of the first reason, I did not look into detail into the sources and how he uses them, but a surface read of the paragraph raises no red flags.
In terms of the second reason, it gets a bit hairier. The idea that they could be considered limbs which are generally covered, and therefore one should not look at them for hanaah, or look at them while saying Shema, is well established. And the extension to require a woman to cover that area because of tznius is not a difficult leap at all. However, I have a few issues with this. And perhaps they are valid complaints, and perhaps not.
He states that "it is fully-accepted Orthodox practice... and as such it is an obligation of everyone not to deviate from this practice." This makes it seem (to me) that he is stating that this is universal practice, for Orthodox people, and so in every place one must conform to this. Especially since he does not mention or stress that there is legitimate basis for not doing so. And by saying that Mishna Berura, Salmas Chaim, and Igros Moshe give this reason, it seems that they are giving this reason in general, for all groups. On closer reading, one can see that the text in Oz VeHadar Levushah can mean that these sources give this as a reason specifically where those are usually covered, but not to endorse the idea that it is therefore a universal requirement. If so, it is a misleading phrasing, and it is quite possible that this was done unintentionally -- but it fits into the general trend of the book to present material in a way that stresses chumra and ignores kullah, and presents sources in a slanted way so that one thinks that this, in its entirety, is what the sources say.
(On further reflection, it is also possible that this statement he makes about a "fully-accepted Orthodox practice" is true for his target audience, which is the more chareidi communities. If so, one should be careful about applying the dictates in the book to other communities, where the assumptions may not hold.)
We should go through each of these sources and see what they do say, and what they do not say.
Mishna Berura basically states that shok is the thigh, not the calf. But in places where it is the derech to go with it covered, it has the law of something which is generally covered, so that, in terms of Shema, a tefach exposed would prevent saying Shema. He does not actually address an "obligation for covering the lower part of the legs." Though it is certainly not a farfetched extension.
The specific Shu"t Salmas Chaim, from Rav Yosef Chaim Zonnenfeld, zatzal, is available from Hebrew Books here and on the next page, here. It is actually pretty short, so it pays to read the next teshuva as well, siman 67, to get a better picture. After reading this, my take is that he indeed agrees that the reason for covering is not because it is shok but because it is an area usually covered (see in the question portion of siman 66). But at the same time, he does not say it is a universal requirement. Rather, in the question portion, it is stated that it is dependent upon the minhag hamakom, and the question then was what the specific minhag was in the chareidi community in Yerushalayim at that time. And in the teshuva, he does not deny that. Thus, he certainly leaves it as something dependent upon local customs. He does not say it is universal Orthodox Jewish practice, and therefore incumbent upon everyone, which is either stated, or strongly implied, in Oz veHadar Levushah. If that is what Oz veHadar is stating, then it would seem to be Rabbi Falk's own supposition.
In terms of that Teshuva in Igros Moshe, a while back, for other reasons, I posted it with a translation here at parshablog. (Read it there.) And the disconnect between the teshuva and the summary in Oz veHadar Levushah was jarring to me.
Firstly, Rav Moshe Feinstein appears to be giving a reason for the strange hakpada of certain individuals, that they insist that their wives and daughters go out in stockings, but do not insist that the stockings are not see-through (he redefines this to be flesh-colored; see inside). That does not mean that he necessarily endorses this requirement upon all individuals. He ends it that "perhaps this is their intent." The intent would either be that they consider it ervah but without hirhur, and therefore a mere covering would suffice; or perhaps, even though it is not ervah, it is more tznius in that it is, after all covered. Therefore, I don't see any reason to assume that Rav Moshe was endorsing this as a universal requirement.
Furthermore, Rav Moshe says that there is some tznius fulfillment in this, because it after all covers. It is certainly possible, and plausible, that he is saying that this is because it is, for them, a place which is usually covered. But other things are possible, such as that within this group, they choose to be more modest, and this is a covering. Thus, Rav Moshe Feinstein never explicitly says that it is because it is an אבר שדרכו להיות מכוסה, and it is (IMHO more than) possible that he is not in fact talking about it -- let alone stating that this is a universal requirement.
But perhaps I am reading the teshuva incorrectly. Read it for yourself inside. Perhaps we can hammer it out in the comment section.
At any rate, I asked an Orthodox Jewish woman about calf-coverings in New York City, since I know nothing about fashion. She told me that there certainly are plenty of Orthodox Jewish women in NYC who do not cover their calves with hosiery.
They would then not be doing something wrong. The fact that they do this would make the calf an אבר whose derech is not to be covered. Therefore, there is no problem saying Shema opposite an uncovered calf. And similarly, it is not required based on tznius. And we could rely on the Mishna Berurah and Rav Yosef Chaim Zonnenfeld for this, that it is based on the specific practice of the women in that locale. (And as stated above, perhaps Rabbi Falk was addressing only the communities where this is not the case. Though if so, declaring it a "fully-accepted Orthodox practice" is unintentionally misleading.)
Aside from any of this, something about this declaration feels circular to me. And I think the question comes down to whether אבר שדרכו להיות מכוסה comes from the bottom up or is dictated from the top down. The fact is that women's clothing styles change organically over time. I would posit that if suddenly a group of religious Jewish women started revealing a specific non-erva area, such as the calf, (and in the secular realm, that area is often not covered) then that would quickly become, by definition, an area on the body which is not usually covered, such that it would be permitted for them not to cover it. This is because it is specifically because it is usually a covered area that it causes hirhur when a man looks at it.
Instead, Oz veHadar Levushah is dictating, from the top down, what appropriate garb should be, based on present styles of women. And it may be true -- since they dress like this, they must cover. But the function of a book like Oz veHadar Levushah is to dictate, from on-high, the appropriate dress. And so people will be dressing like this because Oz VeHadar says it is required, and Oz VeHadar says it is required, (in part) because they are dressing like this. Further, on page 335 (and 336), he decries the pritzus of the legs (including regarding tights) as something which has arisen in the past few decades, and so he is coming to correct this problem. But if this is in fact the way that women's dress has evolved, then this would not seem to be something in need of correcting.