Rotate object by touch using Angles?

I am trying to rotate object using touch, I would like it to move from where it is touched from outer edge to where it gets dragged, while rotating around another object as the pivot. I am only trying to rotate its y axis. I was using angles, but problem is depending where I start the touch it will rotate the wrong way. I am converting all points to screen points.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ScreenControlls : MonoBehaviour
 {
     public GameObject cam;
     public float rotSpeed = 0.5f;
     private Vector3 startRot;
     private Touch initTouch = new Touch();
     private Touch touch;
     public GameObject pivotpos;
     private Vector3 pivotposscreenspace;
     private Vector3 currentpos;
     private float angle;
     private Vector3 initialpos;
     private float originalangle;
 
 
     // Update is called once per frame
     void Update()
     {
         if(Input.touchCount==1)
         {
             touch = Input.touches[0];
             if(touch.phase == TouchPhase.Began)
             {
                 initTouch = touch;
                 initialpos = Camera.main.ScreenToViewportPoint(initTouch.position);
                 originalangle = cam.transform.eulerAngles.y;
             }
             else if (touch.phase == TouchPhase.Moved)
             {
 
                 pivotposscreenspace = Camera.main.WorldToViewportPoint(pivotpos.transform.position);
                 currentpos = Camera.main.ScreenToViewportPoint(touch.position);
                 angle = (Vector3.SignedAngle(currentpos,initialpos, pivotposscreenspace))*rotSpeed;
               
                 cam.transform.eulerAngles = new Vector3(startRot.x,originalangle+angle,startRot.z);
             }
             
            // }
             else if (touch.phase == TouchPhase.Ended)
             {
                 initTouch = new Touch();
             }
         }
 
     }
 }