This is the css of a navigation bar I've made:

header{
    width: 85%;
    display: block;
    margin-left: auto;
    margin-right: auto;}

#nav{
    width: 100%;
    float: left;
    margin: 0 0 3em 0;
    padding: 0;
    list-style: none;
    background-color: #6E6E6E;
    border-radius: 10px;
    border-bottom-right-radius: 50px;
    border-top-left-radius: 50px;}
#nav li{
    display: inline;
    float: left;
    position: relative;}
#nav li a:link, #nav li a:visited{
    display: block;
    padding: 8px 15px;
    text-decoration: none;
    font-weight: bold;
    color: #FFF;
    border-right: 2px solid #FFF;
    font-family: sans-serif;
    font-size: 2.5em;}
#nav li a:hover, #nav li a:active{
    background-color: #454545;}
#nav_edge:hover, #nav_left:active{
    background-color: #454545;
    border-bottom-left-radius: 10px;
    border-top-left-radius: 50px;}
#nav li ul{
    position: absolute;
    left: -9999px;
    white-space: nowrap;
    list-style: none;}
#nav li ul li a:link, #nav li ul li a:visited{
    background-color: #6E6E6E;
    border-top: 2px solid #FFF;}
#nav li ul li a:hover{
    background-color: #454545;}
#nav li:hover ul{
    left: 0em;
    margin-left: -40px;}

How would I go about having a menu "pop up" left of the previously dropped menu when I hover over "about the product"?
Here's the header of the html:

    <header>
        <ul id="nav">
            <li><a id="nav_edge" href="#">Home</a></li>
            <li>
                <a href="#">About</a>
                <ul>
                    <li><a href="#">About us</a></li>
                    <li><a href="#">About the product</a></li>
                </ul>
            </li>
            <li><a href="#">Products</a></li>
            <li>
                <a href="#">Contact</a>
                <ul>
                    <li><a href="mailto:html@troll.com">Email</a></li>
                </ul>
            </li>
            <li><a href="#">Store</a></li>
        </ul>
    </header>
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