The IOR values,if measured correctly,are based on a perfect flat surface.this means ,that the bright reflection curve at 90°is allways white (1) for dielectric materials,on a flat surface.
but if the roughness is increased,then the IOR curve doesent fit anymore for the grazing angle,because the light disburtion gets more and more diffuse.this is even more truth, if the geometry from the object has a microroughness.the structural and the microfacets going hand in hand.the IOR curve is on a very rough surface allmost flat,and if geometry is rough too,its get even darker at the 90°angle,because it gets self shadowing.
the IOR for the material itself stays the same for refraction,but as sayed the reflection (and its curve) its changing with structural an microfacet alterings.
a easy way to get real roughness values are,search a PBR material in the net and load the roughness map from the material you want.if you go with a colorpicker over the map, you can easy get the roughness values from it.or even use the map.
here in my last posting ,you can see,how increased roughness on a metal, is changing the reflection curve.