You are not an idiot. Believe me, a quilter who has not done what you have done at least once, is a rare thing indeed. And I don't think quilters are the only ones. I bet carpenters do it a lot too, hence the carpenters credo "measure twice, cut once". We all know that saying and still we end up with the occasional short, pieced together with scraps piece. I have more than a few borders on quilts with a 1/2" patch in them.