An oldtimer taught me this method several years ago. He called it "The Center of Neutral" method. It is a very accurate method of CG placement and it becomes more accurate if you attach the sling directly at the tail and nose of the plane. It has several drawbacks, though. First, you need to make sure that the thrust line is perfectly level. If it isn't, the CG will be way off from where you measure it to be. Of course, you need to use a level with almost any method to be the most accurate. Secondly, you need a lot of patience waiting for everything to stop swinging. Third, though it might be considered an advantage, you must balance the model laterally first to get it to sit in the sling. I've set my CG using this method and then put the model directly on the CG Machine for comparison and frankly, the only difference was that this method was more accurate as the point on the plumbob was easier to finely mark than the "pads" on the CG Machine. The two results were otherwise virtually identical. The CG Machine had the advantage of being much quicker and easier.