As far as I can tell, Kent didn’t snap to anything; he just eyeballed it.
Try it, you might surprise yourself how close you get it and when it’s a little bit larger, then that would make total sense as well, from a construction point of view.
If you feel uncomfortable with that, you can get it exact by copying the Y- and Z-sizes, but remember,: this is not CAD-modelling and inaccuracies and imperfections are your friend with cases like this!
