Zoom camera to correct FoV based on Rect size

I have a Rect which is drawn on screen and would like my camera to zoom in so that this Rect box fills the screen. I am trying to apply an equation (found here) which calculates the appropriate FoV required to do this. The equation is:

  float camera.fieldOfView = 2 * Mathf.Atan(frustumHeight * 0.5f / distance) * Mathf.Rad2Deg;

The issue is that I'm not too sure what to enter for the frustumHeight. This doesn't seem to be the height of the Rect, at it's much smaller than that required (based on manually changing the FoV to see what looks approximately right).

I was wondering if there is a conversion I need to do somewhere to get the correct height?

Note, the Rect is used to surround a number of objects on screen. It was calculated by finding all of their collider.bounds min and max X and Y values and then uses these values to draw the smallest sized Rect around them (i.e. touching the colliders). As the coordinates used to calculate the Rect were in World Space, is it not right to assume then that the height of the Rect should also be in World Space scale?

Thanks