Uh oh. The next obstacle has come up ... just when I was feeling good about myself for knitting a gauge swatch and adjusting my needle size to achieve the correct stitch gauge. As it turns out, I also need to worry about row gauge. The heel flap of my sock has come out the correct width, but a bit too long!
Investigation reveals that I should've blocked my swatch and then checked gauge in both directions - across the stitches and over the rows. I've surfed around several online articles and Ravelry posts about blocking, and decided I'd better machine wash and machine dry my swatch and then reconsider my gauge. Machine washing and drying is definitely the way to go, I think, because I don't know anyone who's likely to handwash or steam a pair of socks.
So, for now the sock project's on hold... the needles are clicking out a new gauge swatch. Here's hoping there's some good news tomorrow!