Additional comment

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/CommonMaterial.hlsl

 // This is a coarse approximation of computing fresnel0 for a different top than air (here clear coat of IOR 1.5) when we only have fresnel0 with air interface
 real Fresnel0ReajustFor15(real fresnel0)
 {
-    real sqrtF0 = sqrt(fresnel0);
-    return Sq(1.0 - 5.0 * sqrtF0) / Sq(5.0 - sqrtF0);
+    //real sqrtF0 = sqrt(fresnel0);
+    //return Sq(1.0 - 5.0 * sqrtF0) / Sq(5.0 - sqrtF0);
+
+    // Optimization: Fit of the function (3 mad) for range 0.04 (should return 0), 1 (should return 1)
+    return saturate(-0.0256868 + fresnel0 * (0.326846 + (0.978946 - 0.283835 * fresnel0) * fresnel0));
 }
 
 // same as regular refract except there is not the test for total internal reflection + the vector is flipped for processing

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.hlsl

 
         // fresnel0 is deduced from interface between air and material (assume to be 1.5 in Unity, or a metal).
         // but here we go from clear coat (1.5) to material, we need to update fresnel0
+        // Note: Schlick is a poor approximation of Fresnel when ieta is 1 (1.5 / 1.5), schlick target 1.4 to 2.2 IOR.
         preLightData.baseFresnel0.x = Fresnel0ReajustFor15(bsdfData.fresnel0.x);
         preLightData.baseFresnel0.y = Fresnel0ReajustFor15(bsdfData.fresnel0.y);
         preLightData.baseFresnel0.z = Fresnel0ReajustFor15(bsdfData.fresnel0.z);