Just came across the question as I was researching Ray Tracing in UE4 4.26. This may be too late for you @viduttam but just in case someone else comes along after me…
If I interpret the below ArtStation post (h/t Iri Shinsoj) correctly it sounds like Epic re-normalizes the inputs in UE4 to fit the range that Lagarde describes:
“In Unreal Engine the specular is a value between 0 and 1 and is used to scale the amount of specularity on non-metallic surfaces. It has no effect on metals. In most cases just let the value by default (0.5) which is good for 99% of the materials. Most of the materials reflect 4% of light, when looking strait at the surface. Specular value of 0.5 represents 4% reflection, that said, the specular range 0-1 represents 0-8%. Engine handles the fresnel effect, rising the reflectivity at the angles.”