Relationship between Field of View and Zoom

We have found that the relationship between zoom and field of view is denoted by an arctangent function such as FoV=arctan(d/2f) where D is equal to image size (horizontal length in mm, for example) and F is the effective focal length. For sensor payloads with zooming capabilities (such as the X3000) is there a focal length correlating to the zoom factor that could be used in this equation, or is there another way to calculate field of view based off of the numbers given in the background guide?