The Change in SpotLight Intensity with Distance from the center (in degrees).

I’m trying to recreate the Unreal Spotlight with an IES file, which maps degrees from the center to intensity values, but I can’t find anything in the Engine Source or Shader files that tells me how the effects of the spotlight are calculated. Could somebody let me know how the SpotLight Intensity values change with angle from the center, or where I could find that? I’ve looked in:

LightFunctionPixelShader.usf, LightFunctionVertexShader.usf, LightGridInjection.usf, LightMapDesnitySahder.usf, LightShafShader.usf