Calculate the required height for the right amount of scrolling

I’ve found this working example here: Use Vertical Scrollbar to Horizontal Scroll Content. It’s great except that I’d have to give a random height to #fakecontent. I’d like that when I scroll all the way down, I’d just arrive at the end of the horizontal slider. Not too far, not too close.

I tried to find the required ratio with no luck. with / height etc etc

How is finding the correct height of #fakecontent achievable?

$(window).on('scroll', function() {
  $("#realcontent").css("left", -$(window).scrollTop());
});
#realcontent {
  background-color: #333;
  position: fixed;
  top: 5px;
  left: 0;
  width: 2000px;
  color: #fff;
  height: 100px
}

#fakecontent {
  height: 2000px;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="realcontent">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ullam a est maiores fugiat nesciunt, at ad. Tempore odio velit ipsam, laborum explicabo repudiandae aliquid nostrum qui dolorem obcaecati, autem expedita!</div>
<div id="fakecontent"></div>