OT: central limit theorem

I have one of these inexpensive Ikea bookshelves for storing some of my
electronics books:
<https://www.ikea.com/us/en/p/laiva-bookcase-black-brown-40178591/>
I noticed the center shelf was starting to sag a few degrees. :( The assembly manual specifies a weight limit of 33 lbs evenly distributed which seemed like an oddly specific number. So I weighed the books on the shelves, which aren't particularly well organized other than to fully fill the available space widthwise on each shelf.
A random assortment of hardbacks and paperbacks, some are tall and skinny, some are short and fat. And each shelf was clocking in at 33 lbs +/- 2 lbs.
So I guess a heuristic for filling these shelves is just fill 'em up then remove the heaviest book, and de-rate the center shelf by maybe 5- 10 lbs because it's unsupported by a backing.