I have the same question, I have tried to followed your post and followed the answer in my template but it didn't work for me the code is as follows:

$layout_config = json_decode ('{
"two_sidebars": {
"default" : [ "' . ($allowsidebar ? 'span4 offset4' : 'span12') . '" , "span4 offset-8" , "span4" ],
"wide" : [],
"xtablet" : [],
"tablet" : [ "span12" , "span6 spanfirst" , "span6" ]
},
"one_sidebar1": {
"default" : [ "' . ($allowsidebar ? 'span8' : 'span12') . ' has-sidebar1 pull-right" , "span4" ],
"wide" : [],
"xtablet" : [ "' . ($allowsidebar ? 'span8' : 'span12') . ' has-sidebar1 pull-right" , "span4" ],
"tablet" : [ "span12" , "span12 spanfirst" ]
},
"one_sidebar2": {
"default" : [ "' . ($allowsidebar ? 'span8' : 'span12') . ' has-sidebar2" , "span4" ],
"wide" : [],
"xtablet" : [ "' . ($allowsidebar ? 'span8' : 'span12') . ' has-sidebar2" , "span4" ],
"tablet" : [ "span12" , "span12 spanfirst" ]
},
"no_sidebar": {
"default" : [ "span12" ]

when i change the span and offset settings to any other it is not taking effect I am using JA MAGZ Template. Please help

Regards