Basic gradient vert shader weirdness with 2+ objects

Ok so I cobbled together a basic vertical gradient surface shader, all seems to work fine until you use the same material on two objects and then once only one is visible on the screen (presumably the other object is culled) then the measurements change which screws up the gradient position.

I presumed shaders were calculated on an object by object basis so not sure why the others verts would influence each other. Also if anyone could give me some pointers on how to normalise the verts to make the gradient automatically spread from the top to the bottom of the model without manually adjusting it that would be amazing.

 Shader "Gradient/Local" {
 Properties {
     _MainTex ("Base (RGB)", 2D) = "white" {}
     _Color1 ("Bottom Color", Color) = (0,0,1,1)
     _Color2 ("Top Color", Color) = (1,0,0,1)
     _Scale ("Scale", Float) = 1
     _Offset ("Offset", Float) = 0
     _Glossiness ("Smoothness", Range(0,1)) = 0.5
     _Metallic ("Metallic", Range(0,1)) = 0.0
 
 
 }
 SubShader {
     Tags { "RenderType"="Opaque" }
     LOD 200
 
     CGPROGRAM
     #pragma surface surf Standard fullforwardshadows vertex:vert
     #pragma target 3.0
 
     sampler2D _MainTex;
     half _Glossiness;
     half _Metallic;
     fixed4 _Color1;
     fixed4 _Color2;
     fixed  _Scale;
     fixed  _Offset;
 
 
     struct Input {
         float2 uv_MainTex;
         float3 localPos;
 
     };
 
     void vert (inout appdata_full v, out Input o){
         UNITY_INITIALIZE_OUTPUT(Input,o);
         o.localPos = v.vertex.xyz;
     } 
 
 
     void surf (Input IN, inout SurfaceOutputStandard o) {
         half4 c = tex2D(_MainTex, IN.uv_MainTex);
         c.rgb *= lerp(_Color1,_Color2, (IN.localPos.y * _Scale)+_Offset);
         o.Albedo = c.rgb;
         o.Alpha = c.a;
         o.Metallic = _Metallic;
         o.Smoothness = _Glossiness;
 
     }
     ENDCG
 }
 Fallback "Diffuse"
 }