I have a lump in my right lower leg that appears only when I flex my calf muscle. It is painless, pressible, symmetric

If you can feel it as a soft moveable lump, it is probably subcutaneous. If in doubt, an ultrasound could determine if is sub-fascial or intramuscular, and only becomes visible with tightening your calf muscles.
In either case it can be removed surgically. An MRI will confirm it looks benign before proceeding to remove it. If it has any suspicious characteristics, then a FNA, fine needle aspirate should be done before attempting to remove it. If a FNA is done, it should be done by an orthopedic surgical oncology service.
It is highly likely to be a benign lipoma, but if a soft tissue lump is suspected of not being benign, orthopedic surgical oncology is the specialty that should handle it.
A review of litigation for missed diagnosis of soft tissue sarcomas made the following point:
"A second suggestion to arise from these results is the potential “false sense of security” that can surround a sarcoma presentation – especially with new soft tissue “lumps.” Many sarcomas may be subtle or unassuming, and they can be commonly mistaken for benign lesions, such as fatty lipomas. Advanced imaging of suspicious masses can give heightened suspicion for sarcoma based on aggressive imaging features. Commonly, however, practitioners presented with a superficial soft tissue [lump] will forego an MRI in favor of a high degree of subjective clinical diagnostic certainty – a confidence that may be falsely inflated by a lack of experience in dealing with soft tissue masses.
In the Journal of Surgical Oncology 2014;110:919–929
Medical Malpractice and Sarcoma Care—A Thirty-Three Year Review of Case Resolutions, Inciting Factors, and At Risk Physician Specialties Surrounding a Rare Diagnosis

Lipomas are benign fatty tumors that occur under the skin and less commonly within the muscle. They are associated with mild discomfort and a lump underneath the skin. It is always best to talk to an expert plastic surgeon to determine whether the lipoma is possibly a cyst or a tumor or hernia. We would have to remove the lipoma to be sure that it is not something else and we usually do this by sending it to a pathologist for evaluation. If a lipoma is painful, then an exam is needed to make sure it is not compressing on a nerve or whether the diagnosis is different. Aftercare for a lipoma includes gentle compression and Plato’s Scar Serum twice daily on the wound.

Best,
Dr. Karamanoukian
Realself100 Surgeon