I brought up my own hamstring/calf pain with my teacher (I'm studying massage, neuromuscular therapy, but we also learn active isolated stretching (AIS), sports injuries, etc). She said that pain is often felt in the back of the legs when the problem is really in the front (strengthen the back, stretch the front). She gave me two AIS stretches for the quads and hip flexors. Essentially, you're lying on your side pulling the leg back for a quad stretch and then moving the pelvis forward (so you're not in a straight line of hips to knees anymore) and stretching the hip flexors. Just hold for two seconds, release and stretch.