I had just figured it out and came back here to let you know. The problem was indeed, as you say, selecting the wrong thing. I was indeed selecting the box line... so it worked for the dimension... but not for the calculator. The VERTEX at the corner of the box has to be selected for it to appear in the calculator as the vertex is what is constrained... not the line.
Anyway, the problem is solved. I am having a lot of fun with constraints. You can do most anything with them. It was a bit of a pain at first but like everything else, once you get used to it you wonder how you made all those parts fit together with the right clearances without it... haha.
Thanks Again Henry,