How many rows should a heel flap be?
if you are working across 36 heel stitches, do 36 rows). OR knit until you can fold the heel flap in half diagonally, and one side meets the other side. Folded diagonally one side meets the other, therefore the flap is long enough.
What is a sock gusset?
Gusset/Instep: This is the part of the sock that runs from the heel into the main part of the foot. It can normally be seen as a triangular shaped area where shaping either side of the foot decreases the number of stitches after a heel flap.