offset half window height

I’m trying to write 50% of body height as the addition: .scrollTop() + 50%} rather than .scrollTop() + 200} So when the mouse leaves the #tail div snaps back to half the window height.

    .bind('mouseleave', function(){
     $  ('#wrapper').unbind('mousemove');

          $  ('#tail').animate({top:$  ('body').scrollTop() + 50%},300)
});

Whats the correct way to add a percent value?

Thanks

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