Sometimes, it's the little things.
In this case, the oddball of yarn I used was just the right amount for the heel.
Pattern: Sock Recipe: A Good, Plain Sock by Stephanie Pearl Mc-Phee, in the book Knitting Rules.
Yarn: Malabrigo Sock in colorways Turner (leg and foot) and Playa (heel and toe).
Link to Ravelry project: A Good, Plain Sock