Get an object to trail behind you

using UnityEngine;
using System.Collections;

public class KeepDistance : MonoBehaviour {

	// Use this for initialization
	void Start () {
	
	}
	//Restricts the position of this object to be within a distance of "distance" of "center"
    public float distance = -40;
    public Transform center;
	// Update is called once per frame
	void Update () {
		
		float dst = Vector3.Distance(center.position, transform.position);
        if (dst > distance)
        {
            Vector3 vect = center.position  - transform.position;
            vect = vect.normalized;
            vect *= (dst-distance);
            transform.position += vect;
	
	}
}
}

I need to keep the object trailing behind the player. When I have a positive number for the variable distance the object stays in front of me at the same distance. When I have a negative number for the variable distance the object blinks in front and in back of the player.

I can’t quite remember but I believe that Vector3.Distance is always positive, to confirm this use print(“dst”). If this is the case make distance = 40. Also remember that Vector3.Distance acts like a radius, this means that while the object my be 40 away from the player this doesn’t have to be behind them.