Shader Alpha

Hey,

I have pretty straightforward question but I'm completely lost in shaders :/ I'm trying to make a cutoff layer using stencil but I don't want to make that layer solid, but use alpha so it'll cover graphics behind it in only 50% for example (best if I could dynamically choose it). I have cutoff, I have black covering layer, but when I change alpha from 255 to 254 layer dissapears completly. Any ideas how to change it?

Code (CSharp):

1. Shader "Custom/Stencil NotEqual" {
2.     Properties {
3.         _Color ("Color", Color) = (1,1,1,0)
4.         _MainTex ("Albedo (RGB)", 2D) = "white" {}
5.         _Glossiness ("Smoothness", Range(0,1)) = 0.5
6.         _Metallic ("Metallic", Range(0,1)) = 0.0
7.         _Cutoff("Cutoff", Range(0,1)) = 0.5
8.     }
9.     SubShader {
10.         Tags {  "Queue" = "Transparent"
11.                  "RenderType"="Transparent"}
12.         Blend SrcAlpha DstAlpha
13.         LOD 200
14.  
15.         Stencil {
16.             Ref 1
17.             Comp NotEqual
18.         }
19.        
20.         CGPROGRAM
21.         #pragma surface surf Lambert alpha:fade
22.  
23.         // Use shader model 3.0 target, to get nicer looking lighting
24.         #pragma target 3.0
25.  
26.         sampler2D _MainTex;
27.  
28.         struct Input {
29.             float2 uv_MainTex;
30.         };
31.  
32.         half _Glossiness;
33.         half _Metallic;
34.          float _Cutoff;
35.         fixed4 _Color;
36.  
37.         void surf (Input IN, inout SurfaceOutputStandard o) {
38.             // Albedo comes from a texture tinted by color
39.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
40.             o.Albedo = c.rgb;
41.             // Metallic and smoothness come from slider variables
42.             o.Metallic = _Metallic;
43.             o.Smoothness = _Glossiness;
44.             o.Alpha = c.a;
45.             clip(c.a - _Cutoff);
46.         }
47.         ENDCG
48.     }
49.     FallBack "Diffuse"
50. }
51.  

Thanks a lot!