What is the rule to convert the 'mm' notation to the 'optical zoom' notation? I searched a bit and found this one:

$$\text{Optical zoom} = \frac{\text{Maximum focal length}}{\text{Minimum focal length}}$$

For example a 18–55mm lens would have a 3x optical zoom, and a 18–200mm lens would have a 11x optical zoom. Is that right?