Stop moving background image after a certain X position is reached

I have a section (#intro) with a background image that has horizontal movement on scroll.

$(window).bind("load resize scroll",function(e) {
var y = $(window).scrollTop();

$("#intro").filter(function() {
    return $(this).offset().top < (y + $(window).height()) &&
           $(this).offset().top + $(this).height() > y;
}).css('background-position', + parseInt(+y / 10) + '% 50%'); ));

What I would like to happen is the background position to move until 80% is met for the X position; at which point the number will not increase. I want the stopping point to be '80% 50%', but if a user scrolls back up it will count down again.

How can I achieve this?