percentageFromCfi(cfi) {
    if (this._locations.length === 0) {
      return null;
    } // Find closest cfi


    var loc = this.locationFromCfi(cfi); // Get percentage in total

    return this.percentageFromLocation(loc);
  }
Get a percentage position in locations from an EpubCFI
@param {EpubCFI} cfi
@return {number}