Absolutely positioning the span with a 'top' position of 10px means the 'tooltip' will always appear 10px from the top of the page. You can try something like this:
a.tooltp:hover span
{display:inline;
position:relative;
left:50px;
-moz-box-shadow: 0px 0px 8px 4px #666;
-webkit-box-shadow: 0px 0px 8px 4px #666;
box-shadow: 0px 0px 8px 4px #666;
border:2px solid white; background:#000; color:white;}
Where the positioning is relative to the "box" div and sets the left of the tool tip 50px from the left of the box div. You may want to consider using javascript to exactly how what the value should be for that, it will depend on how wide the box div is.