<style>
    #scroll {
      display:none;
      position:fixed;
      top:0;
      right:20px;
      z-index:500;
      padding:3px 8px;
      background-color:#2187e7;
      color:#fff;
      border-radius:3px;
    }
    #scroll:after {
      content: " ";
      position: absolute;
      top:50%;
      right:-8px;
      height:0;
      width:0;
      margin-top:-4px;
      border:4px solid transparent;
      border-left-color:#2187e7;
    }
    </style>

<div id='scroll'></div>

<script type='text/javascript'>
//<![CDATA[
var scrollTimer = null;
$(window).scroll(function() {
   var viewportHeight = $(this).height(),
       scrollbarHeight = viewportHeight / $(document).height() * viewportHeight,
       progress = $(this).scrollTop() / ($(document).height() - viewportHeight),
       distance = progress * (viewportHeight - scrollbarHeight) + scrollbarHeight / 2 - $('#scroll').height() / 2;
    $('#scroll')
        .css('top', distance)
        .text(' (' + Math.round(progress * 100) + '%)')
        .fadeIn(100);
    if (scrollTimer !== null) {
        clearTimeout(scrollTimer);
    }
    scrollTimer = setTimeout(function() {
        $('#scroll').fadeOut();
    }, 1500);
});
//]]>
</script>