After measuring Chad’s foot, I cast on 36 stitches. (Should be one inch bigger than “medium-large”)
I think the pattern might be off in the heel flap: it says to knit 16 rows, which is ridiculously too many. Did 10-12 IIRC, and picked up 8 stitches for the gusset.
may have made them slightly too short. 🙁