This study looks at how heat loss from the limbs effects thermo regulation in humans.
The segmented body diagram shows that:
Head, neck and torso includes about 39.82% of the total surface area.
The Arms and hands account for about 21%
The legs account for about 31%
Venting the legs can have a big effect on temperature regulation because of their relatively large surface area and because the leg muscles can generate lots of heat. This report from Cornell University states that the heat generated by muscles can be 40 times that of all other tissue.