How to load scene when async.progress is 0.9?

I want to load my scene when async.progress is 0.9,but first of all,i want to run my loading screen text at least once,but i dont know why it doesnt work

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
  
 public class LoadScreenScript : MonoBehaviour
 {
     public Text DoYouKnow;
     public Text loadingText;
  
  
     public int imageNumber;
     public float letterPause;
  
     public string sceneToLoad;
     string messageToAnimate;
     // Use this for initialization
  
     void Start ()
     {
         messageToAnimate = ".......";
         loadingText.text = "Loading";
  
         changeDoYouKnowText ();
         StartCoroutine (LoadingScreen ());
     }
    
     // Update is called once per frame
     void Update ()
     {
        
     }
  
     void changeDoYouKnowText()
     {
         imageNumber = Random.Range (1, 5);
         Debug.Log (imageNumber);
         if (imageNumber == 1)
             DoYouKnow.text = "hello mikel";
         else if (imageNumber == 2)
             DoYouKnow.text = "hello erick";
         else if (imageNumber == 3)
             DoYouKnow.text = "hello kevin";
         else if (imageNumber == 4)
             DoYouKnow.text = "hello world";
     }
  
     IEnumerator LoadingScreen ()
     {
         AsyncOperation async = Application.LoadLevelAsync (sceneToLoad);
         Debug.Log(async.progress);
         async.allowSceneActivation = false;
         do{
             foreach (char letter in messageToAnimate.ToCharArray())
             {
                 loadingText.text += letter;
                 yield return 0;
                 yield return new WaitForSeconds (letterPause);
             }
             loadingText.text = "Loading";
             Debug.Log(async.progress);
         }while(async.progress<0.9);
         async.allowSceneActivation = true;
     }
  
  
 }