//get actual current height of nav_inner div
var divh = document.getElementById('nav_inner').offsetHeight;

//find the whole pixel half of that height
var halfHeight="-"+Math.round(divh/2)+"px";

//center the contents inside the "nav" div
if(document.getElementById('backLink')){
  document.getElementById('inner_nav').style.top="142px";
}else{
  document.getElementById('inner_nav').style.top="160px";
}
document.getElementById('nav_inner').style.top=halfHeight;