#Top_bar .menu > li > a:after question.

I want to reduce the width percentage on the #Top_bar .menu > li > a:after line to 50% but I can't find a way to center it once it's reduced. This is my current style. What would you recommend I do?

This is for http://cherrylake.com/cltf2/home/root-opt-3/

#Top_bar .menu > li > a:after {
content: "";
height: 2px;
width: 50%;
text-align:center;
position: absolute;
left: 0;
top: 64px;
z-index: 203;
opacity: 0;
filter: alpha(opacity=0); }

I appreciate your help.

Thank you

Erik