I guess that the formula for getting the focal length is so basic that it's not mentioned most of the time. The formula is: "Focal Length = Mirror Diameter * Focal Ratio". Very quickly it becomes one of the natural things that optics workers just remember - kind of like the basic formula for resistance, voltage and current in electronics - E=I*R - that you learn in electronics in one of the first lessons. Focal ratio is exactly what it means - the ratio between the diameter of the optic and the focal length of that optic.