My method is to knit a tension swatch in the round and pull it over the ball of my foot to see if it stretches to be how snug I'd like. Knowing the ball of my foot is 9.5 inches I can then work out how many stitches I'll need at each point up the leg. My mistake here was not working both cables into the swatch. The travelling stichwork actually seemed to stretch nearly the same as the plain stockinette.
Some knitters advocate a two-swatch system: work one swatch in pattern, and another in stockinette, then average the "stretch" of the two to come up with the stitch count.