How do I rotate shoulder bone up and down using Rotate and Space.World independend on model orientation

Question

I wanted to move my character's arms up and down in a straight way.

The Rotation Gizmo tool can do exactely that.

Playing with the wheel of the Rotation Gizmo tool in the attached screenshot (marked with red arrows in my screenshot), I could move the arms down in a straight way:

I can do the same using the following script:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class MyPlayer : MonoBehaviour
{
    public Transform Shoulder;
private Quaternion _Rotation;
private float _f = 0f;

private void OnGUI()
{
    if (GUI.Button(new Rect(0, 70, 50, 30), "+"))
    {
        _f += 1;
        Shoulder.Rotate(_f, 0, 0, Space.World); ;
    }
}

}

This works perfectly fine, but no longer when I rotate the main model (=the parent of the shoulder).

How could I make this script independend on the main transform's orientation?

Thank you!

Edit:

I would guess that I have to take the main transform (=parent) orientation into account in order to subtract its global rotation from the calculations, but not really knowing which values I should choose, I didn't find the correct formula. However, FYI, I tried the following:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class MyPlayer : MonoBehaviour
{
    public Transform Shoulder;
public Transform Mother;

private float _f = 0f;

private void OnGUI()
{
    if (GUI.Button(new Rect(0, 70, 50, 30), "+"))
    {
        _f += 1;
        _f -= Mother.transform.rotation.eulerAngles.x;
        Shoulder.Rotate(_f, 0, 0, Space.World); ;
    }
}

}