Pixel perfect sampling in surface shader

I'm creating procedural texture, with something like:

 int texSize = 20;
 Texture2D tex = new Texture2D (texSize, texSize, TextureFormat.RGBA32, false, false);
 tex.filterMode = FilterMode.Point;
 
 for (int i = 0; i < tex.width; i++)
 {
     for(int j = 0; j < tex.height; j++)
     {
         if(i == 2 && j == 3)
              tex.SetPixel(i, j, new Color(1.0f, 0.0f, 0.0f));
         if(i == 2 && j == 4)
              tex.SetPixel(i, j, new Color(1.0f, 0.0f, 0.0f));
     }
 }
 
 tex.Apply();

then in shader i want to sample pixel from this texture

 Shader "Custom/Terrain" 
 {
 Properties 
 {
     _TexData ("HexData", 2D) = "white" {}
 }
 SubShader 
 {
     Tags { "RenderType"="Opaque" }
     LOD 200
     //Cull off
     
     CGPROGRAM
     #pragma surface surf Lambert finalcolor:mycolor
     #pragma target 3.0
 
     sampler2D _TexData;

     void surf(Input IN, inout SurfaceOutput o) 
     {
         o.Albedo = grass; 
         o.Alpha = 1.0;
     }
   
     void mycolor (Input IN, SurfaceOutput o, inout fixed4 color)
     {
         color.rgb = tex2D(_TexData, float2(    3 / 20.0f, 4 / 20.0f));// returns correctly red
         //or
         color.rgb = tex2D(_TexData, float2(    3 / 20.0f, 5 / 20.0f));// returns white
     }
 
     ENDCG
 } 
 FallBack "Diffuse"
 }

i'm getting right values when sampling 3 / textSize, 4 / textSize, but wrong for 3 / textSize, 5 / textSize.

i'm saving texture to disk for testing and it looks good:

is there a way to sample texture by pixel, or fix my pixel to text coord calculation?