Menü zentrieren

[Kurtis]

wie kann ich das submenü zentriert anzeigen?? anstatt mit pixel!!
momentan ist die angabe noch in pixel (left position, top position)

thanks kurtis

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Menus = new Object();
var curMenu = null;

MakeMenu = function(name,left,top,rollimg,rollout,rollover,rolldiv){
 this.name = name;
 this.left = left;
 this.top = top;
 this.timer = null;
 this.rollimg = rollimg;
 this.rollout = newImage(rollout);
 this.rollover = newImage(rollover);
 this.rolldiv = (rolldiv != null) ? rolldiv : null;
 this.obj = name+'Object';
 eval(this.obj+'=this');
}
MakeMenu.prototype.show = function(){
 clearTimeout(this.timer);
 getStyle(this.name).visibility = 'visible';
 imageSwap(this.rollimg,this.rollover,this.rolldiv);
 curMenu = this.name;
}
MakeMenu.prototype.hide = function(){
 this.timer = setTimeout(this.obj+'.hideIt()',500);
}
MakeMenu.prototype.hideIt = function(){
 getStyle(this.name).visibility = 'hidden';
 imageSwap(this.rollimg,this.rollout,this.rolldiv);
}
MakeMenu.prototype.startMenu = function(){
 var html;
 if(ns4){
  html = '<layer name="'+this.name+'" left="'+this.left+'" top="'+this.top+'" z-index="1000" visibility="hidden" ';
 }else{
  html = '<div id="'+this.name+'" style="position:absolute; left:'+this.left+'px; top:'+this.top+'px; z-index:1000; visibility:hidden" ';
 }
 html += 'onMouseOver="Menus.'+this.name+'.show()" onMouseOut="Menus.'+this.name+'.hide()">';
 return html;
}
hideCur = function(){
 if(curMenu != null){
  Menus[curMenu].hideIt();
  curMenu = null;
 }
}
endMenu = function(){
  return ns4 ? '</layer>' : '</div>';
}
createMenu = function(name,left,top,rollimg,rollout,rollover,rolldiv){
 Menus[name] = new MakeMenu(name,left,top,rollimg,rollout,rollover,rolldiv);
}

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// object name | left position | top position | rollover image name | normal image src | rollover image src | rollover div name (optional)

createMenu('submenu1', 20,  41, 'img_1', 'images/img_out.gif', 'images/img_over.gif');
createMenu('submenu2', 120, 41, 'img_2', 'images/img_out.gif', 'images/img_over.gif');
createMenu('submenu3', 220, 41, 'img_3', 'images/img_out.gif', 'images/img_over.gif');

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