Atten includes the shadow value premultiplied in. You have to pick out the code that does the attenuation from the code that does the shadows before they get multiplied together. The code's in AutoLight.cginc...
Shader Forge has, I think, both a light position and object position nodes. Hook both of those up to a distance node. Divide that by a VALUE (// light range) node. Plug that into a Power node A slot, and plug a new VALUE (// attenuation) into the B slot. Multiply by Light Color Clamp 0-1 Into Emission. If you want it to…