FLs are tough to deal with responsively because they have no concept of what’s going with other elements. If I have to, I usually link them to a page-level element (bumper).
If you can make a bumper at the page level behave, you can id it “bump”, id your REU as fl_left and use this code:
$(window).resize(function(){
s = $('#bump').parent().position();
$('#fl_left').css('left' , s.left + 'px');
});
Don’t forget to run the inside of the function on page load.
Depending on your setup you might have to use .closest() to find the first r-box.