Trying to make a custom cutout shader

Hi,

I'm looking for some shader help, it's not really my strong point. I'm trying to wright a custom shader that slightly modifies the standard shader in cutout rendering mode. Rather than use the albedo textures alpha for cutting out the mainTex I pass in a different texture and calculate its UV in screenspace. I want to use this screenspace texture to cut out the main texture so the player isn't obscured. So far so good, my problem is actually setting up my shader so it actually cuts out the alpha.