[C# attached] [screenshots attached] determining delta rate for UI casting bar

Hello, dear community! I would like to create a casting bar UI, and have found a wonderful tutorial here: http://www.youtube.com/watch?v=wtQ1ylRLZsA. However, I decided to write a script myself. It works almost as intended: the bar moves from start position to the end position smoothly no matter what time interval I set (1 second or 10 seconds doesn't matter). But it moves slightly more than it should.

Because of this - because the bar arrives always at exact time to the wrong position - it seems the problem lies in the way "rate" variable is calculated. Please have a look, for your convenience I am attaching C# script and screenshots. Your comments are welcome!

 using UnityEngine;
 using System.Collections;
 
 public class castingbarscript : MonoBehaviour {
 
 
     public GameObject castbar;
     public Vector3 startpos;
     public Vector3 endpos;
     public Vector3 Resultpos;
 
     public float timer1;
     public bool doing;
     public float rate;
 
     // Use this for initialization
     void Start () {
     
         startpos = castbar.transform.position;
     //    startpos = new Vector3 (0,0,0);
         endpos = new Vector3(castbar.transform.position.x - castbar.GetComponent<RectTransform>().rect.width, castbar.transform.position.y,castbar.transform.position.z);
     //    endpos = new Vector3 (155, 0, 0);
 
         Resultpos = startpos - endpos;
 
         doing = false;
         timer1 = 1f;
         rate = (Resultpos.x/timer1)*Time.deltaTime;
     }
 
 
     // Update is called once per frame
     void Update () {
         Debug.Log (castbar.transform.position);
         if (Input.GetKeyDown (KeyCode.J)) {
             doing = true;
 
         }
 
         //reset
         if (Input.GetKeyDown (KeyCode.L)) {
             doing = true;
             castbar.transform.localPosition = new Vector3 (0,0,0); 
             timer1 = 1f;
         }
 
 
         if (doing) {
             timer1 -= Time.deltaTime;
             if(timer1 >= 0){
             castbar.transform.position = new Vector3 (castbar.transform.position.x - rate,castbar.transform.position.y,castbar.transform.position.z); 
             //    castbar.transform.position = Vector3.Lerp(startpos,endpos, 0.5f);
         
             }
         }
     }
 }