#back-top {  
position: fixed;  
width: 50px;  
height: 100px;  
z-index: 6;  
right: 25px;  
bottom:0;  
background-color: transparent;  
display: block;  
background-position: initial initial;  
background-repeat: initial initial;  
  }

<div id="back-top">  
<center>  
<a title="Вверх" id="backop" href="#top">  
<img src="/img/go_up.png" />  
</a>  
<font color="#000000" size="3">
<span id="pix">0</span>%</font>  
</center>  
  </div>  

  <script>  
  $(window).scroll(function () {  
var s = $(window).scrollTop();  
var f = $(document).height() - $(window).height();  
var d = s / f * 100;  
var p = Math.round(d);  
$("#pix").text(p);  
  });  
  </script>