How to use this script for a depleting fuel bar as you thrust with a rocket

  using UnityEngine;
  using System.Collections;
 
  public class Progress : MonoBehaviour {
      public float barDisplay; //current progress
      public Vector2 pos = new Vector2(20,40);
      public Vector2 size = new Vector2(60,20);
      public Texture2D emptyTex;
      public Texture2D fullTex;
  
      void OnGUI() {
      //draw the background:
      GUI.BeginGroup(new Rect(pos.x, pos.y, size.x, size.y));
          GUI.Box(new Rect(0,0, size.x, size.y), emptyTex);
      
          //draw the filled-in part:
          GUI.BeginGroup(new Rect(0,0, size.x * barDisplay, size.y));
              GUI.Box(new Rect(0,0, size.x, size.y), fullTex);
          GUI.EndGroup();
      GUI.EndGroup();
  }
  
  void Update() {
          //for this example, the bar display is linked to the current time,
          //however you would set this value based on your desired display
          //eg, the loading progress, the player's health, or whatever.
          barDisplay = Time.time*0.05f;
 //       barDisplay = MyControlScript.staticHealth;
     }
 }

So this code is from another question response on this site itself. Im a newb, so i could use some help. If i just use it as is, attach it to my rocket gameobject and set the float fuel = barDisplay it only depletes the bar after the fuel bar hits 0, which coincidentally is the point in my code that i disable the thrusting movement function and invoke my Death() after a few seconds just in case the player is on a friendly platform like the launcher in my game.