I'm in the midst of making a lightbox style pop-up for a mailing list sign up, but I want the pop-up to position to the center of the visible page, not just the center of the whole document; if the user scrolls to the bottom of the page and clicks to sign up, I want it to appear in the center of the screen.

I'm assuming jQuery/JS will be the best way to go for this; here's my current CSS code which works fairly well but the div needs to be pushed down into the visible space dynamically for smaller screens.

.my-div{

    width:960px;

    height:540px;

    position:absolute;

    top:50%;

    left:50%;

    margin-left:-480px;

    margin-top:-270px;

    z-index:60;

    display:none;

}