I finished the socks!
And here's what I did wrong:
(apart from messing up the lace pattern that trails down the foot, looking stunning throughout, by one stitch precisely, every time I ripped back and tried to line it up right - yay inexperience with socks of any kind let alone flap and gusset, grrrr - such that I finally decided to leave it out altogether since it is freezing out now anyway.)
(at which point I thanked the weather for turning so I could justify giving up that particularly nice detail.)
I didn't check my gauge.
Oh, I checked that I was getting the right stitches per inch. I just didn't check that I was getting the right number of inches for my foot. Even though I know my foot is oddly narrow.
So the heel and ankle are a little on the floopy side, while the foot is snug, that being the point at which I discovered the problem.
You can kind of see the problem in profile - the way the gusset comes all the way down as I kept on decreasing, and how much narrower the foot is than the ankle.
It doesn't look as though the heel floopiness is sufficient to surge out of the back of my shoes though, so I have decided to ignore this and just make a few notes for the next time I make these socks.
And I will make them again, if I make socks at all - and we know the likelihood of that (astronomically high) - because it is such a nice pattern*, and the yarn! oh my goodness, the yarn. Unbelievably soft and lustrous.
* this is a Ravelry link, which is probably going to be depressing for anybody non-Ravelly who wants to see the details. e-mail if if you are that person and I'll find out what I can for you!