gistfile1.js

// https://github.com/twitter/bootstrap/issues/675#issuecomment-6184243

$('#feedback-modal').modal({
        backdrop: true,
        keyboard: true
    }).css({
       'width': function () { 
           return ($(document).width() * .9) + 'px';  
       },
       'margin-left': function () { 
           return -($(this).width() / 2); 
       }
});