As I asked the question, I offer a simple solution for a simplified ratio with low-poly and max-feature.
Area is a metric of 2D surface, so I evaluate the area and check the difference between original model. As the compress ratio goes up, the plot curve goes down, eventually. So the last biggest area is the last chance. In the triceratops case, 0.7 is a best ratio to keep max feature with a relative low-poly.
Also, pay attention to toes of triceratops, the toes geometry feature are kept.
