Scaling the Particle System to ScreenSize?

I have been trying to scale the particle System to match different screen sizes, and noticed some odd effects (which i would conclude to be the result of the Camera.main.WolrdPointtoViewPoint)
I attached the following script to the particle system :

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

public class BackgroundController : MonoBehaviour {

	private float xmin;
	private float ymin;
	private float xmax;
	private float ymax;

	// Use this for initialization
	void Start () {
		ymin = Camera.main.ViewportToWorldPoint(new Vector3(0,0,this.transform.position.z - Camera.main.transform.position.z)).y;	
		xmin = Camera.main.ViewportToWorldPoint(new Vector3(0,0,this.transform.position.z - Camera.main.transform.position.z)).x;
		xmax = Camera.main.ViewportToWorldPoint(new Vector3(1,1,this.transform.position.z - Camera.main.transform.position.z)).x;
		ymax = Camera.main.ViewportToWorldPoint(new Vector3(1,1,this.transform.position.z - Camera.main.transform.position.z)).y;

		this.transform.position = new Vector3(
			(xmin+xmax)/2f
			,ymax
			,this.transform.position.z);

		ParticleSystem ps = this.GetComponent<ParticleSystem>();
		Vector3 psBox = new Vector3((Mathf.Abs(xmin)+Mathf.Abs(xmax)/2),1f,1f);
		ParticleSystem.ShapeModule pshape = ps.shape;
		pshape.box = psBox;
	}
	
	// Update is called once per frame
	void Update () {
		
	}
}

Everything works fine, but the particle System is not scaled correctly, its not wide enough to fit the whole screen. At the moment I have trouble understanding “WHY” this is happening. Maybe anyone has tried something alike.
I’m using Unity 5.6.1f1.

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