For a realistic anatomy, what is the length of mermaid tail compared to her torso?

For realistic anatomy, figure the tale would be slightly longer than the legs on a human because the flippers are broader than feet -- if that makes sense. Most people, not all but most are 7 1/2 heads high with the legs beginning four heads down from the top of the head. Figure mermaids as eight heads with the tail beginning four heads down approximately.