trying to understand layout_config

I thought I had the logic down for the mainbody content php files.

The standard is :

  // Layout configuration

  $layout_config = json_decode ('{  

    "two_sidebars": {

      "default" : [ "span6"         , "span6"             , "span3"               , "span3"           ],

      "wide"    : [],

      "xtablet" : [ "span8"         , "span4"             , "span4"               , "span4 spanfirst"           ],

      "tablet"  : [ "span12"        , "span12 spanfirst"  , "span6"               , "span6"           ]

    },

    "one_sidebar": {

      "default" : [ "span9"         , "span3"             , "span3"             ],

      "wide"    : [],

      "xtablet" : [ "span8"         , "span4"             , "span4"             ],

      "tablet"  : [ "span12"        , "span12 spanfirst"  , "span12"            ]

    },

    "no_sidebar": {

      "default" : [ "span12" ]

    }

  }');

which is basically
content 6
maincol 6
sidebar1 3
sidebar2 3
(with maincol over the 2 sidebars)

I want it to be
content 8
maincol 4
sidebar1 2
sidebar2 2

But if the maincol isn't there and there's only one sidebar the content would be 10. how would I address that?