Não consigo alinhar menu dropdown ao centro da tela (CSS+HTML)

[jrsilverio]

No caso, eu gostaria que alinha-se apenas as opções, o fundo continua 100% da tela.

 

#menu {background: rgba(0, 0, 0, 0.8);color: #FFF;height: 51px;padding-left: 0px;z-index: 300;left: 0;top: 0;right: 0;margin: 0;}
#menu ul, #menu li {margin: 0 auto;padding: 0;list-style: none;}
#menu ul {width: 100%;}
#menu li {float: left;display: inline;position: relative;}
#menu a {display: block;line-height: 51px;padding: 0 14px;text-decoration: none;color: #FFFFFF;font-size: 14px;}
#menu a.dropdown-arrow:after {content: "\002B";margin-left: 5px;}
#menu li a:hover {color: #CCCCCC;background: #F2F2F2;}
#menu input {display: none;margin: 0;padding: 0;height: 51px;width: 100%;opacity: 0;cursor: pointer}
#menu label {display: none;line-height: 51px;text-align: center;position: absolute;left: 35px}
#menu label:before {font-size: 1.6em;content: "\2261"; margin-left: 20px;}
#menu ul.sub-menus{height: auto;overflow: hidden;width: 170px;background: #FFFFFF;position: absolute;z-index: 99;display: none;}
#menu ul.sub-menus li {display: block;width: 100%;}
#menu ul.sub-menus a {color: #191919;font-size: 16px;}
#menu li:hover ul.sub-menus {display: block}
#menu ul.sub-menus a:hover{background: rgba(0, 0, 0, 0.8);color: #F2F2F2;}

@media screen and (max-width: 800px){
	#menu {position:relative}
	#menu ul {background:#111;position:absolute;top:100%;right:0;left:0;z-index:3;height:auto;display:none}
	#menu ul.sub-menus {width:100%;position:static;}
	#menu ul.sub-menus a {padding-left:30px;}
	#menu li {display:block;float:none;width:auto;}
	#menu input, #menu label {position:absolute;top:0;left:0;display:block}
	#menu input {z-index:4}
	#menu input:checked + label {color:white}
	#menu input:checked + label:before {content:"\00d7"}
	#menu input:checked ~ ul {display:block}
}

<!-- MENU -->

<nav id='menu'>
  <input type='checkbox' id='responsive-menu' onclick='updatemenu()'><label></label>
  <ul>
    <li><a href='http://'>X</a></li>
    <li><a class='dropdown-arrow' href='http://'>Produtos</a>
      <ul class='sub-menus'>
        <li><a href='http://'>X</a></li>
        <li><a href='http://'>X</a></li>
        <li><a href='http://'>X</a></li>
        <li><a href='http://'>X</a></li>
      </ul>
    </li>
    <li><a class='dropdown-arrow' href='http://'>Serviços</a>
    <ul class='sub-menus'>
        <li><a href='http://'>X</a></li>
        <li><a href='http://'>X</a></li>
        <li><a href='http://'>X</a></li>
        <li><a href='http://'>X</a></li>
      </ul>
    </li>
    <li><a class='dropdown-arrow' href='http://'>Financeiro</a>
    <ul class='sub-menus'>
        <li><a href='http://'>X</a></li>
        <li><a href='http://'>X</a></li>
      </ul>
    </li>
    <li><a href='http://'>X</a></li>
    <li><a href='http://'>X</a></li>
    <li><a href='http://'>X</a></li>
    <li><a href='http://'>X</a></li>
  </ul>
</nav>