What would you suggest for someone who has "athletic" calves but skinny ankles?Easy! Identify the size and stitch count you need for the foot/ankle, and also for the calf. (Use the negative ease rule in both cases - find the circumference that is about 10-15% smaller than your actual circumference.
At the simplest level, the process is this: If you're working top-down, cast on for larger size and work decreases as you work down the leg to ensure that you hit the smaller size by the time you're at the ankle. You can then finish the sock following the instructions for the smaller size. And if you're working toe-up, work the toe, foot and heel turn for the smaller size. Once you're back in the round and working the leg, work increases to get to the larger stitch count, and finish the leg.
How you actually do the shaping depends on your own leg. If the shape change is gradual from ankle to calf, change the stitch count over that distance. If your legs change shape quickly, change the stitch count quickly, too!
For example, let's say that your leg needs 72 stitches, and your foot needs 60 stitches. If working top-down, you will cast on 72, and need to have decreased to 60 by the ankle. If working toe-up, you will work the foot and heel on 60 stitches, and you'll need to increase to 72 for the leg.
That's 12 stitches of shaping.
If shaping gradually: Assuming your leg is at least 6 inches long, increase/decrease 2 stitches every inch or so on the leg. You can eyeball this, no need to be super-precise about it. After all, knitting stretches!
If shaping "quickly": work the shaping in one round in the bottom round of the leg.
I'm doing a lot of writing on this topic right now - stay tuned for news about that being published...