When you want to hang something on the wall (picture frame, etc) put a tiny dab of toothpaste on the item where the nail will sit. Line up the item where you want it on the wall, leaving a tiny gap where the toothpaste is. When it's where you want it, press the item all the way against the wall. This will leave a tiny toothpaste mark on the wall, telling you EXACTLY where to drive the nail. It works like a DREAM! No measuring, no multiple attempts, no time consuming methods. I've used this method for YEARS and it works so much better than anything else. I find eyeballing it in this way is better than even measuring, because rooms are rarely flawlessly proportioned, so even if you measure exactly, it won't look quite right.