Specular from alpha in custom shader

Hi, I've been learning about writing my own shaders, particularly from Kenneth Lemmers youtube tutorials which are great. I currently have a fake BRDF shader with diffuse and normal map included. But I'm having trouble adding specular from the diffuse texture's alpha. I know how to do it using the BlinnPhong surface but unsure with the current code I have here. I've removed any lines that attempt to use specular in the hopes to hopefully prevent confusion. :)

 Shader "Custom/PP_BRDF" 
 {
     Properties 
     {
         _ColorTint ("Colour Tint", Color) = (125,125,125,1)
         _MainTex ("Albedo", 2D) = "white" {}
         _BumpMap ("Normal Map", 2D) = "bump"{}
         _Ramp2D ("BRDF Ramp", 2D) = "gray" {}
     }
     
     
     SubShader 
     {
         Tags { "RenderType"="Opaque" }
         LOD 200
         
         CGPROGRAM
         #pragma surface surf Ramp
         #pragma target 3.0
 
         float4 _ColorTint;
         sampler2D _MainTex;
         sampler2D _BumpMap;
         sampler2D _Ramp2D;
 
         
 
         struct Input 
         {
             float4 color : Color;
             float2 uv_MainTex;
             float2 uv_BumpMap;
         };
         
         half4 LightingRamp (SurfaceOutput s,half3 lightDir, half3 viewDir, half atten)
         {
             float NdotL = dot(s.Normal, lightDir);
             float NdotE = dot(s.Normal, viewDir);
             
             //Diffuse Wrap
             float diff = (NdotL * 0.5) + 0.5;
             float2 brdfUV = float2(NdotE, diff);
             float3 BRDF = tex2D(_Ramp2D, brdfUV.xy).rgb;
             
             float4 c;
             c.rgb = BRDF;
             c.a = s.Alpha;
             return c;
         }
         
 
         void surf (Input IN, inout SurfaceOutput o)
         {
             half4 c = float4(0.5,0.5,0.5,1)/ tex2D (_MainTex, IN.uv_MainTex)/2;
             IN.color = _ColorTint;
             o.Albedo = tex2D(_MainTex, IN.uv_MainTex) * IN.color/c.rgb * 2;
             o.Alpha = c.a;
             
             
             o.Normal = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }