Open the Knitter's Almanac to page 118, study the pictures, read the pattern, do a diagram of the leg part.
look at the picture of the half finished sock, the wide part is around the calf, the decreasing part is the sides of the heel, the small bit is the top of the foot.
I'd do a diagram if I could figure out a way to draw and post, I can't.
Then measure your leg, or the leg of happy recipient, or some passing stranger.
Length from the back of the knee to the top of your heel, where the Achilles tendon mets the calf, my measurement is 10 inches, err on the side of less this length is easy to add to by adjusting the ribbing. much later
|Pinned to the apron of a loom|