Min-width/min-height problem

Hi All,

Sorry for my bad english, I’m having a huge problem on centering a DIV


#myDiv {
    position: absolute;
    background-color: #333;
    width:100%;
    min-width: 960px;
    height: 100%;
    min-height:550px;
    left:50%;
    top:50%;
    margin-left: - /* I need a function that gives me half of the div width here */ px;
    margin-top: - /* I need a function that gives me half of the div height here */ px;
}

Can anyone please HELP?

Thanks por your attention!!!

Best regards!