CalculateCameraLimits

    public static void CalculateLimits(Camera aCam, Bounds aArea, out Rect aLimits, out float aMaxHeight)
    {
        // Half the FOV angle in radians
        var angle = aCam.fieldOfView * Mathf.Deg2Rad * 0.5f;

        // half the size of the viewing frustum at a distance of "1" from the camera
        Vector2 tan = Vector2.one * Mathf.Tan(angle);
        tan.x *= aCam.aspect;

        // the center point of the area and it's ectents
        // the center point is taken from the bottom center of the bounding box
        Vector3 dim = aArea.extents;
        Vector3 center = aArea.center - new Vector3(0, aArea.extents.y, 0);

        // the maximum distance the camera can be above the area plane for each direction
        Vector2 maxDist = new Vector2(dim.x / tan.x, dim.z / tan.y);

        // actual distance of the camera above our plane
        float dist = aCam.transform.position.y - center.y;

        // the max movement range around the center of the plane
        dim.x *= 1f - dist / maxDist.x;
        dim.z *= 1f - dist / maxDist.y;

        // maxumum worldspace y coordinate the camera can be moved to
        aMaxHeight = center.y + Mathf.Min(maxDist.x, maxDist.y);

        // the min and max x and z coordinates the camera can be at the current distance.
        aLimits = new Rect(center.x - dim.x, center.z - dim.z, dim.x * 2, dim.z * 2);
    }