Using science (that is, gauge and actual measurements of your foot), we've knit to our measurements and should be near the center of the ankle when we've completed the gusset increases. If you want, you can check this using the method Bordhi outlines in Personal Footprints: draw a line down the middle of your leg, try on your sock, and see if it reaches. You might need to add a few rows to make up for foot expansion. Now it's time to mark where your leg will be.
(You might have figured out that the point of this series was to explore whether I could successfully make socks like those in Personal Footprints for Insouciant Sock Knitters, but without everything I dislike about those socks: the round toe, the "footprint" that doesn't adapt well to different gauges, and especially the annoying trial-and-error process. But the steps that deal with opening up the leg are the parts that I really, really like about the book.)
Sock patterns are usually written with the assumption that people's ankles are about the same circumference as the ball of their foot. For most of my family, it's pretty close, but here is where you can customize. The base of my ankle is just a bit bigger around than the base of my foot, so I'm going to plan for 72 stitches in circumference rather than the 68 stitches I had at the ball.
So what I do now is run a lifeline through the stitches where I will later put the leg of the sock. If my ankle will be 72 stitches, then I need to run a lifeline through 36 stitches, centered on the top. Following Bordhi's instructions, I knit another round, marking a stitch that I will later cut and unravel for the leg opening. Then I knit the next round and run another lifeline through the 36 stitches above my first life line.
I experimented with a more familiar method -- knitting my leg stitches with a bit of waste yarn, which I later removed to knit the leg -- but the end result isn't as nice. And in this instance, I recommend following Bordhi's instructions exactly. If you can't get your hands on a copy of Personal Footprints, you can get the general idea from the Houdini Sock pattern and from Bordhi's YouTube videos.