How to perform counter-clockwise page turn effect, based on existing clockwise code?

Hello, guys! I'm trying to create page flip effect in Unity, using UI system. I've found such code:

     using UnityEngine;
     using System.Collections;
     public class CurlEf : MonoBehaviour {
 
     public Transform _Front;
     public Transform _Mask;
     public Transform _GradOutter;
     public Vector3 _Pos;
 
 
     void Start()
     {
         _Pos = transform.position;
     }
 
     void LateUpdate()
     {
         // transform.position = _Pos; // To fix position of back image
         // transform.eulerAngles = Vector3.zero;
 
         Vector3 pos = _Front.localPosition;
         float theta = Mathf.Atan2 (pos.y, pos.x) * 180.0f/ Mathf.PI; // angle between X and Y?
         if (theta <= 0.0f || theta >= 90.0f) return;
 
         float deg = -(90.0f - theta) * 2.0f;
         _Front.eulerAngles = new Vector3(0.0f, 0.0f, deg);
 
         // Move mask to "cut" front and back images
         _Mask.position = (transform.position + _Front.position) * 0.5f;
         _Mask.eulerAngles = new Vector3(0.0f, 0.0f, deg*0.5f);
 
         // Move gradient to create effect of volume
         _GradOutter.position = _Mask.position;
         _GradOutter.eulerAngles = new Vector3(0.0f, 0.0f, deg * 0.5f + 90.0f);
 
         // Next to make immobilize back image
         transform.position = _Pos; // To fix position of back image
         transform.eulerAngles = Vector3.zero;
     }
 }

It uses Mask to perform flip effect and works perfectly, but works only in a clockwise direction (kind of turn page back to prev page). What I'm trying to do is to perform a counter-clockwise animation, based on this code. But I don't understand how to do this.

Check this video for detailed view https://www.youtube.com/watch?v=CUW3fGEK9as and this link for whole project: http://gnupart.tistory.com/entry/Unity3D-Simple-Page-Curl-Effect

Could you please, help me? At least, could someone explain what is theta and how to calculate deg for my purposes?