Re: the set-up. I suppose I should have mentioned that both ends have already been cut at 45 degree miters, obviously the key objective is to place the first piece so it's not too short, not too long where it mates with the second piece.
If it really is merely accurate measuring corner to corner, no problem. I had imagined, for example, that two mitered scrap pieces might be cut and each tacked into a corner, as if they represented the second piece to be applied and the last piece...and then measurements taken from their innermost corners. Unnecessarily complex it would seem. Thanks for the good ideas.