I'm showing you this picture to make the point that slight errors or mistakes in measurement are NOT A big deal with the foam core/veneer method. Because I hadn't glued the foam core together precisely,
the top of this wall set in a little too far. I couldn't get the vertical strip on the corner to fit right. So I glued on a small piece of scrap to bring it out a bit. If the corner of the wall was out too far, I could have shaved off a thin layer of the foam core.  The beauty of separating the strength of the structure from its appearance is that it becomes easy to compensate in one area to benefit the other.