Anyone interested in this as to where it came from, get out the 1948 edition of "Soil Mechanics in Engineering Practice" by Terzaghi and Peck. Art. 54, page 413, 1948 edition John Wiley & Sons. They have a rather compete history and explanation of these tables, so many out there. In my review the 1" settlement is not mentioned. Hey you youngsters, that was the textbook for my first Soil Mechanics course, the most popular at the time.