I was peering at the copy of this painting in “Dress in Eighteenth-Century Europe,” and it’s actually a slightly clearer copy — and what I noticed there, which you can’t see in the scan I have on my site, is that there is a very faint line of the spangle decoration that angles up from the top of the bottoms & loops to that folded back corner. Aha! So it IS angled, and I think actually closes a bit further over than I currently have it — off to monkey with my current pattern.
And, thank god for protractors — even so, drawing the wave pattern for the spangles has been waaaay too fiddly!