Sniper scope zoom if crosshair is not in the middle of the screen

Hi, As you could understand I'm working on FPS game and I need to have a look into sniper scope but the t$$anonymous$$ng is that my crosshair can be anywhere in the screen w$$anonymous$$ch makes some troubles for sniper aiming.

Here is my view before the sniper aim.

And my view after.

What I'm doing is decreasing camera's field of view but I get the unneeded offset (as you can see on the screenshot).

Could you please help me to figure out how to calculate new camera position or what do I need to do in t$$anonymous$$s situation?

         var aimTransform = weapon.transform.Find("aim_pos");
         mainCamera.transform.position = aimTransform.position;
         //mainCamera.transform.rotation = aimTransform.rotation;
 
         //TODO: calculate "offset" to stay point at the same object as before zoom in
         // distance to first $$anonymous$$t
         // mainCamera.ScreenPointToRay(new Vector3
         var position = new Vector3(crosshairPosition.x, Screen.height - crosshairPosition.y, 0);
         Ray rCast = mainCamera.ScreenPointToRay(position);
         //Debug.LogWarning("Ray param = " + rCast);
         Debug.DrawLine(rCast.origin, rCast.origin + rCast.direction * 10000, Color.yellow,600);
         
         RaycastHit $$anonymous$$tInfo;
         bool wasHit = Physics.Raycast(rCast, out $$anonymous$$tInfo);
         float distance;
         if (wasHit)
         {
             Debug.Log(string.Format("Hit {0} distance = {1}", $$anonymous$$tInfo.collider.name, $$anonymous$$tInfo.distance));
             distance = $$anonymous$$tInfo.distance;
         }
         else
         {
             Debug.LogWarning("Not $$anonymous$$t. take farClipPlane");
             distance = mainCamera.farClipPlane;
         }

         Vector3 wp1 = mainCamera.ScreenToWorldPoint(new Vector3(crosshairPosition.x, crosshairPosition.y, distance));
         Debug.Log(wp1);
         mainCamera.fieldOfView = zoomInFow;
         Vector3 wp2 = mainCamera.ScreenToWorldPoint(new Vector3(crosshairPosition.x, crosshairPosition.y, distance));
         Debug.Log("new " + wp2);
 
         var worldOffsetX = wp1.x - wp2.x;
         var worldOffsetY = wp1.y - wp2.y;
         var worldOffsetZ = wp1.z - wp2.z;
         Debug.LogWarning(string.Format("World offset x={0} y={1}", worldOffsetX, worldOffsetY));
 
 mainCamera.transform.localPosition = new Vector3(mainCamera.transform.localPosition.x + worldOffsetX, mainCamera.transform.localPosition.y + worldOffsetY, mainCamera.transform.localPosition.z);
 

Thank you, Ihor.