How to center a vertical dropdown menu

Question

I am working on a new wordpress site website and running into a CSS problem with the dropdown menus. Currently they are left-aligned to the main menu or parent menu item. My preference is for them to be centered on the main menu or parent menu item.

Here is a link to the site in development: http://centraltexasdistrict.com/welcome/

Here is a link to an image, which gives an idea of what I'm looking for or trying to achieve: http://centraltexasdistrict.com/wp-content/uploads/CTD-menu.jpg

Here is the current css I'm working with:

    .menu-main-container {
    margin-left: auto;
    margin-right: auto;
    width: 690px;
    padding: 0 0 0 20px;
    height: 35px;
    z-index: 10;
    line-height: 35px;
    }

    .menu-main-menu-container ul li ul{ text-align:center; }

    .main-navigation {
    clear: both;
    margin: 0 auto;
    max-width: 1080px;
    min-height: 36px;
    position: relative;
    }

    #navbar {
    display: block;
    height: 36px;
    width: 100%;
    margin-top: -27px;
    z-index: 10;
    position: absolute;
    }

    #site-navigation {
    padding: 0px;
    height: 30px;
    z-index: 10;
    line-height: 30px;
    float: left;        
    }

    ul.nav-menu,
    div.nav-menu > ul {
    margin: 0;
    padding: 0px;
    }

    .nav-menu li {
    display: inline-block;
    position: relative;
    }

    .nav-menu li a {
    display: block;
    font-family: 'Lato', sans-serif;
    line-height: 30px;
    color: #aaaaaa;
    font-weight: 400;
    text-align: center;
    text-transform:uppercase;
    text-decoration: none;
    letter-spacing: 1px;
    float: left;
    font-size: 12px;
    padding: 30px 7px 0px 7px;
    margin-bottom: 5px;
    }

    .nav-menu li:hover > a,
    .nav-menu li a:hover,
    .nav-menu li:focus > a,
    .nav-menu li a:focus {
    color: #ffffff;
    background-color: #353535;
    }

    .nav-menu .sub-menu,
    .nav-menu .children {
    background-color: #e07726;
    padding: 0px;
    position: absolute;
    z-index: 99999;
    overflow: hidden;
    clip: rect(1px, 1px, 1px, 1px);
    margin-top: 65px;
    }

    .nav-menu .sub-menu ul,
    .nav-menu .children ul {
    border-left: 0;
    left: 100%;
    top: 0;
    }

    .sub-menu {
    text-align: center;
    width: 100%;
    left: auto; 
    }       

    .sub-menu li {
    display: inline-block;
    width: auto;
    padding: 0px;
    }

    .sub-menu li a {
    display: block;
    height: 24px;
    font-family: 'Lato', sans-serif;
    line-height: 24px;
    font-weight: 400;
    text-align: left;
    text-decoration: none;
    text-transform: none;
    letter-spacing: 1px;
    float: left;
    font-size: 11px;
    padding: 2px 10px 2px 10px;
    color: #ffffff;
    white-space: nowrap;
    }

    .sub-menu li:hover > a,
    .sub-menu li a:hover,
    .sub-menu li:focus > a,
    .sub-menu li a:focus {
    background-image: none;
    color: #ac5411;
    background-color: transparent;
    }

    ul.nav-menu ul a,
    .nav-menu ul ul a {
    color: #fff;
    margin: 0;
    }

    ul.nav-menu li:hover > ul,
    .nav-menu ul li:hover > ul,
    ul.nav-menu .focus > ul,
    .nav-menu .focus > ul {
    clip: inherit;
    overflow: inherit;
    height: inherit;
    width: inherit;
    }

    .nav-menu .current_page_item > a,
    .nav-menu .current_page_ancestor > a,
    .nav-menu .current-menu-item > a,
    .nav-menu .current-menu-ancestor > a {
    color: /* #333333 */;
    }

Answer 1

A quick solution could be to give the submenu a specified width:

ul.nav-menu li:hover > ul, .nav-menu ul li:hover > ul, ul.nav-menu .focus > ul, .nav-menu .focus > ul {
  clip: inherit;
  overflow: inherit;
  height: inherit;
  width: 200px; /* width for the submenu */
  left: 50%; /* position it 50% to the left */
  margin-left: -100px; /* negative margin half of with to center it */
}

Or use translate if you dont want to have a specified width (not for all old browsers and you may need some prefixes):

ul.nav-menu li:hover > ul, .nav-menu ul li:hover > ul, ul.nav-menu .focus > ul, .nav-menu .focus > ul {
  clip: inherit;
  overflow: inherit;
  height: inherit;
  transform: translate(-50%, 0);
  left: 50%;
}

Answer 2

.menu-main-menu-container ul li ul{
text-align:center;
}

Add the above code in your stylesheet to center drop down list of menu bar