Unity depth shader not giving correct values

I did the following shader (looked up and changed for my needs):

// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

Shader "Custom/DepthShader"
{
        SubShader
        {
            Pass
            {
                CGPROGRAM

                #pragma vertex vert
                #pragma fragment frag
                #include "UnityCG.cginc"

                uniform sampler2D _CameraDepthTexture;
                uniform half4 _CameraDepthTexture_TexelSize;

                struct input
                {
                    float4 pos : POSITION;
                    half2 uv : TEXCOORD0;
                };

                struct output
                {
                    float4 pos : SV_POSITION;
                    half2 uv : TEXCOORD0;
                };


                output vert(input i)
                {
                    output o;
                    o.pos = UnityObjectToClipPos(i.pos);
                    o.uv = MultiplyUV(UNITY_MATRIX_TEXTURE0, i.uv);
                    // why do we need this? cause sometimes the image I get is flipped. see: http://docs.unity3d.com/Manual/SL-PlatformDifferences.html
                    #if UNITY_UV_STARTS_AT_TOP
                    if (_CameraDepthTexture_TexelSize.y < 0)
                            o.uv.y = 1 - o.uv.y;
                    #endif

                    return o;
                }

                fixed4 frag(output o) : COLOR
                {
                    float depth = DECODE_EYEDEPTH(tex2D(_CameraDepthTexture, o.uv));
                    return depth;
                }

                ENDCG
            }
        }
}

What i want is to get depth map with actual distances, not within 0-1 range. Main script is showing color scene but inside i'm checking what's the distance to center of screen:

    private Shader _shader;
    private Shader shader
    {
        get { return _shader != null ? _shader : (_shader = Shader.Find("Custom/DepthShader")); }
    }

    private Material _material;

    private Material material
    {
        get
        {
            if (_material == null)
            {
                _material = new Material(shader);
                _material.hideFlags = HideFlags.HideAndDontSave;
            }
            return _material;
        }
    }

    private void Start()
    {   GetComponent<Camera>().depthTextureMode = DepthTextureMode.Depth;
    }

    private void OnRenderImage(RenderTexture src, RenderTexture dest)
    {
 
        RenderTexture buffer = new RenderTexture(
                               Screen.width, Screen.height,
                               0,                            // No depth/stencil buffer
                               RenderTextureFormat.ARGBFloat,   // Standard colour format
                               RenderTextureReadWrite.Linear // No sRGB conversions
                           );
        Graphics.Blit(src, buffer, material);

        // main scene
        Graphics.Blit(src, dest);
        RenderTexture.active = buffer;           // If not using a scene camera
        Texture2D outputTex = new Texture2D(Screen.width, Screen.height, TextureFormat.RGBAFloat, false);
        outputTex.ReadPixels(
                  new Rect(0, 0, Screen.width, Screen.height), 
                  0, 0,                          
                  false
        );
        outputTex.Apply();
        Color[] depthMap = outputTex.GetPixels();
        
        Color color = depthMap[Screen.width * Screen.height / 2 + Screen.width / 2];
        Debug.Log("distance to center pixel is: " + color[0]);


        Object.Destroy(outputTex);

    }

So here: Debug.Log("distance to center pixel is: " + color[0]); I'm trying to check correctness of my depth values by checking center pixel value. In the scene I put camera then after 25 meters small wall then again after 25 meters big wall (parallel to small one). So camera is looking at the center of small wall. I am expecting the center pixel to be the small wall so depth value should be ~25.

But what I get from Debug.Log is something around 999.8341. If camera is positioned closer i start seeing more or less reasonable values.

I decided saved those values into file as matrix and here is what it is showing around the center:

Is my approach correct or did I make some mistake?

EDIT: The scene image looks like this: