They realized that 4 was a constant so they multiplied the SFM constant by 4 to get the RPM for a 1" tool (200 SFM X 4 = 800 RPM) and multiplied the SFM by 8 (200 SFM X 8 = 1600 RPM) for a 1/2" tool. A 1/2' tool is one of the most common sizes of end mill used in the metalcutting industry.