Positioning element at center of screen

Answer 1

If you have a fixed div just absolute position it at 50% from the top and 50% left and negative margin top and left of half the height and width respectively. Adjust to your needs:

div {
    position: absolute;
    top: 50%;
    left: 50%;
    width: 500px;
    height: 300px;
    margin-left: -250px;
    margin-top: -150px;
}