non riesco a centrare in modo orizzontalemente la barra di navigazione

questo è codice :

.indentmenu{
font: bold 13px Arial;
width:557px; /*leave this value as is in most cases*/
overflow: hidden;
margin-top:50px;
text-align:center;

}

.indentmenu ul{
margin: 0;
padding: 0;
float: left;
width: 589px; /*width of menu*/
border: 1px solid #564c66; /*dark purple border*/
border-width: 1px 0;
background: black url(menu/bg.jpg) center center repeat-x;
}

.indentmenu ul li{
display: inline;
}

.indentmenu ul li a{
float: left;
color:#000; /*text color*/
padding: 5px 50px;
text-decoration: none;
border-right: 1px solid #564c66; /*dark purple divider between menu items*/
}

.indentmenu ul li a:visited{
color:white;
}

.indentmenu ul li a:hover, .indentmenu ul li .current{
color:#00F; /*text color of selected and active item*/
padding-top: 6px; /*shift text down 1px for selected and active item*/
padding-bottom: 4px; /*shift text down 1px for selected and active item*/
background: black url(menu/bg1.jpg) center center repeat-x;
}