AFAIR light reflected from a plane surface is plane-polarised in the plane of the surface. This means that the orientation of the CPL filter for maximum reduction of reflections will depend on he surface that is producing the reflections. If you turn it so you reduce the reflections from a vertical pane of glass (a shop window for example), you should find that you have to turn it another quarter turn to get rid of reflections from a horizontal surface, i.e. a calm lake or a glass-topped table.
The blue light from the sky is also polarised (because it is reflected from dust particles high in the atmosphere) and the polarisation is at a maximum at right-angles to the direction of the sun, so the amount of darkening you can achieve depends very much on the direction in which you point the camera with respect to the sun.
In either case, just twiddle the filter until you get the best effect.
