Unity comes with a helper method for generating a unique asset paths called AssetDatabase.GenerateUniqueAssetPath. The documentation for this method is rather minimal but the code example below should help you understand how to use it a little better.

[MenuItem("CBX/Test")]
public static void Test()
{
    // outputs "Assets/Rules.xml" if "Assets/Rules.xml" file does not already exist.
    // if "Assets/Rules.xml" already exists it outputs "Assets/Rules 1.xml"  
    Debug.Log(AssetDatabase.GenerateUniqueAssetPath("Assets/Rules.xml"));

    // expects path to start with "Assets/" & outputs a console error in the console "pathName.find("assets/") == 0"  
    // then outputs "Rules.xml" 
    Debug.Log(AssetDatabase.GenerateUniqueAssetPath("Rules.xml")); 
}