Unity_Addons/Assets/Editor/Editor AddOn/Utility/TransformUtility.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public static class TransformUtility
{
    public static float GetGlobalIndex(this Transform transform)
    {
        float retVal = transform.GetSiblingIndex() + 1;
        transform = transform.parent;

        while (transform != null)
        {
            retVal /= 10;
            retVal += transform.GetSiblingIndex() + 1;
            transform = transform.parent;
        }

        return retVal;
    }
}