Screen Coordinates not correct in Shader

My project uses Single Pass Rendering and a Windows Mixed Reality Headset. I have a shader, which is set to a new material "ppMaterial", which in turn is then used in

Code (CSharp):

1. void OnRenderImage(RenderTexture source, RenderTexture destination)
2. {
3.     Graphics.Blit(source, destination, ppMaterial);
4. }

The shader is just supposed to draw circles at the screen edges, so

Code (HLSL):

1. Shader "Custom/ScreenShader" {
2.     Properties {
3.     }
4.     SubShader {
5.         Pass {
6.             CGPROGRAM
7.             #pragma fragment frag
8.             #pragma vertex vert
9.             #include "UnityCG.cginc"
10.  
11.             uniform sampler2D _MainTex;
12.             half4 _MainTex_ST;
13.  
14.             struct appdata
15.             {
16.                 float4 vertex : POSITION;
17.                 float2 uv : TEXCOORD0;
18.             };
19.             struct v2f
20.             {
21.                 float4 vertex : SV_POSITION;
22.                 float2 uv : TEXCOORD0;
23.                 float4 screenPos : TEXCOORD1;
24.             };
25.  
26.             float getDist(float2 isIn, float2 pos2){
27.                 return sqrt((isIn.x - pos2.x)*(isIn.x - pos2.x) + (isIn.y - pos2.y)*(isIn.y - pos2.y));
28.             }
29.  
30.             float4 NearBoundary(float2 isIn, v2f i)
31.             {
32.                 float2 pos2 = float2(0.5,0.5);
33.                 float2 pos3 = float2(0,0.5);
34.                 float2 pos4 = float2(0.5,0.0);
35.                 float dist2 = getDist(isIn, pos2);
36.                 float dist3 = getDist(isIn, pos3);
37.                 float dist4 = getDist(isIn, pos4);
38.                 if(dist2<0.1){
39.                     return float4(0,0.5,0.5,1);
40.                 }
41.                 if(dist3.x<0.3){
42.                     return float4(0.5,0,0.5,1);
43.                 }
44.                 if(dist4.x<0.3){
45.                     return float4(0.5,0,0.5,1);
46.                 }
47.                
48.                 return tex2D(_MainTex, UnityStereoScreenSpaceUVAdjust(i.uv, _MainTex_ST));
49.             }
50.  
51.             v2f vert (appdata v)
52.             {
53.                 v2f o;
54.                 o.vertex = UnityObjectToClipPos(v.vertex);
55.                 o.screenPos = ComputeScreenPos(o.vertex);
56.                 o.uv = v.uv;
57.                 return o;
58.             }
59.  
60.             half4 frag(v2f i) : SV_Target {
61.                 float2 screenPos = i.screenPos;
62.                 return NearBoundary(screenPos, i);
63.             }
64.             ENDCG
65.         }
66.     }
67. }

But this does not work, the dots are not drawn at the center and the borders, respectively. How to get screen coordinates in the (0,1)-range?