center on responsive site

Can you possibly let me know how you would tweak this code to make the ‘Scroll to Top’ link center on all platforms? I have tried tweaking this and can center it on mobile, but that just knocks it off center on larger platforms.

jQuery(document).ready(function(){
    			jQuery(\"body\").append(\"<div id=\\\"scroll_to_top\\\"><a href=\\\"#top\\\">$text</a></div>\");
    			jQuery(\"#scroll_to_top a\").css({	'display' : 'none', 'z-index' : '9', 'position' : 'fixed', 'top' : '100%', 'width' : '$width', 'margin-top' : '-30px', 'right' : '50%', 'margin-left' : '-50px', 'height' : '20px', 'padding' : '3px 5px', 'font-size' : '14px', 'text-align' : 'center', 'padding' : '3px', 'color' : '#$color', 'background-color' : '#$background_color', '-moz-border-radius' : '5px', '-khtml-border-radius' : '5px', '-webkit-border-radius' : '5px', 'opacity' : '.8', 'text-decoration' : 'none'});

Your help would be much appreciated!

Thx

https://www.ads-software.com/extend/plugins/scroll-to-top/