-webkit-transform:  rotateX( -90deg ) translateZ( -webit-calc(100vh - 50vw) );

var responsive_calculations = function () {
    if ( $("#cube").hasClass("show-front") ) {
        $(".bottom").css({
            width:  winWidth,
            height: winWidth,
            transform:  "rotateX( -90deg ) translateZ(" + winHeight - winWidth_half + "px)"
        });
    }
};

responsive_calculations();
$(window).resize(function () {
    responsive_calculations();
});