For anyone wondering how a "professional" would do this, you might use a 3D CAD program like SolidWorks, Inventor, or FreeCAD. You'd make a single 2D sketch, draw some lines in box shapes, and put some constraints on those boxes. Free/unbound dimensions would read out the distances that you needed to know. The whole thing would take less than five minutes for someone with beginner-level skills (i.e., me), and maybe a…
But as another commenter highlighted: that meant a learning curve not only for me, but for my wife as well. And then there was the challenge of sharing it with everyone else via email, IM, etc. Having it open in the browser means it's easy to share, and fast and easy to load and show to people anywhere (which is handy during construction).