You should study Dubcat's link, which is very good, and actually does opposite of his example :- )
You don't need to use reflection map most of the time and the roughness should be represented in glossiness slot instead (as is in article).
In my bellow example my white ceiling paint is diffuse texture and glossiness map. Nothing else, reflection is white. Glossiness slot dictates both blurriness and intensity (the blurrier they are, the more intensity they loose)