shader cg blending two textures

Hi all;

I'm learning cg shaders at the moment and I've managed to blend two textures together by following some intro tutorials. What I want to do is blend one texture into another - in my case, snow into rock. So far I have a slider that allows me to manually adjust the blending of the snow into rock textures (in the object inspector). Is there a way of eliminating the use of the slider and manually set how much of texture 1 blends into texture 2? (using code within the shader only). Any advise will be appreciated.

Code:

 Shader "Custom/something" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _Texture2 ("Blend Texture 1", 2D) = "white" {}
         
         //http://answers.unity3d.com/questions/232106/shader-blending-multiple-textures-in-cg-code.html
         _Blend1 ("Blend between _MainTex and Texture2", Range (0, 1) ) = 0 
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 400
         
         CGPROGRAM
         #pragma surface surf Lambert
         #pragma target 3.0
         //#pragma surface surf BlinnPhong
 
         sampler2D _MainTex;
         sampler2D _Texture2;
         float _Blend1;
 
         struct Input {
             float2 uv_MainTex;
         };
 
         void surf (Input IN, inout SurfaceOutput o) {
             //half4 c = tex2D (_MainTex, IN.uv_MainTex);
             fixed4 mainCol = tex2D(_MainTex, IN.uv_MainTex);
             fixed4 texTwoCol = tex2D(_Texture2, IN.uv_MainTex);
             fixed4 output = lerp(mainCol, texTwoCol, _Blend1);
             o.Albedo = output.rgb;
             o.Alpha = output.a;
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }