How to use RotateAround with swipe gesture

Hi, I need to rotate my camera around a fixed point (0,0,0) when dragging. I managed to get it to work with a mouse click using:

 void Update()
 {
         CameraMovement();
 }

 void CameraMovement()
 {       
      MainCamera.transform.RotateAround(Vector3.zero, Vector3.up, Input.GetAxis("Mouse X") * CameraRotateSpeed);
 }

Which works as I expect with a mouse, you click, you drag, it rotates around the centre, and it stops when you let go of the mouse button. On a touch device, however, it jumps around depending on where you tap. It's hard to explain, but if I tap the centre of the screen, the camera will jump to a position and then rotate around as you swipe. But it seems the tap locations are fixed. If you touch the centre again, the camera jumps back to the same position it did before.

After some Googling, I change it to this:

 void Update()
 {
     if (Input.touchSupported)
     {
         foreach (var touch in Input.touches)
         {
             CameraMovement(touch.fingerId, MainCamera.ScreenToWorldPoint(touch.position), touch.phase);
         }
     }
     else
     {
         if (Input.GetMouseButtonDown(0))
         {
             CameraMovement(10, Camera.main.ScreenToWorldPoint(Input.mousePosition), TouchPhase.Began);
         }
         if (Input.GetMouseButton(0))
         {
             CameraMovement(10, Camera.main.ScreenToWorldPoint(Input.mousePosition), TouchPhase.Moved);
         }
         if (Input.GetMouseButtonUp(0))
         {
             CameraMovement(10, Camera.main.ScreenToWorldPoint(Input.mousePosition), TouchPhase.Ended);
         }
     }
 }

 void CameraMovement(int fingerId, Vector3 pos, TouchPhase phase)
 { 
     switch(phase)
     {
         case TouchPhase.Began:
             break;
         case TouchPhase.Moved:
             MainCamera.transform.RotateAround(Vector3.zero, Vector3.up, pos.x * CameraRotateSpeed);
             break;
         case TouchPhase.Ended:
             break;
     }
 }

But with this, when you touch and swipe or whatever, the camera will always move to an x position of 2.0 and if you hold it, it just moves ever so slightly around 2.0 to 3.0 constantly.

What I'm trying to achieve here, is you swipe to rotate the camera around, so if I swipe from left -> right, the camera will rotate around the 0,0,0 point left -> right and if you swipe from Right -> Left, it will rotate around point 0,0,0 from Right -> Left.