code: function() {
          var resolution = this.getServerResolution();
          return this.serverResolutions ?
              OpenLayers.Util.indexOf(this.serverResolutions, resolution) :
              this.map.getZoomForResolution(resolution) + (this.zoomOffset || 0);
      }
docs: Method: getServerZoom
  Return the zoom value corresponding to the best matching server
  resolution, taking into account <serverResolutions> and <zoomOffset>.
  
  Returns:
  {Number} The closest server supported zoom. This is not the map zoom
  level, but an index of the server's resolutions array.