Take a look at Fiddle . [Edited]
You need to get the coordinates of the mouse, and then subtract the offset of the element to get the relative value.
HTML:
<div class="test"></div>
JS:
$(document).on('mousemove','.test', function(e){
    var offset = $(this).offset();
    var relativeX = e.pageX - offset.left;
    var relativeY = e.pageY - offset.top;
    var wide = $(this).width();
    var tall = $(this).height();
    var percentX = (relativeX*100)/wide;
    var percentY = (relativeY*100)/tall;
    console.log(percentX+", "+percentY);
});
CSS
.test{
  background-color: #000000;
  width: 50%;
  height: 50px;
  margin-left: 40px;
  margin-top: 20px;
}