function ContentMiddlePos()
{
    delta = document.documentElement.offsetHeight - 800;
    if (delta < 0 && delta != oldDelta)
    {
        height = 250 + delta;
        oldDelta = delta;
        if (height < 170)
        {
            height = 170;
        }
        if (height > 250)
        {
            height = 250;
        }
        document.getElementById('header').style.height = height + 'px';        
        return true;
    }
    else if (delta != oldDelta)
    {
        document.getElementById('header').style.height = '250px';
        return true;
    }
}
var height;
var delta;
var oldDelta = 0;
window.onload = ContentMiddlePos;
window.onresize = ContentMiddlePos;