Can someone explain to me precisely how the filter factor works. Today was the first day I've ever used a filter. I used hoya 25a and the filter factor is 3 (2 stops). When I put the filter on, the light meter picked up the 2 stops automatically. I don't need to open up more stops do I?

Also, with my polarizer, a hoya linear, the filter factor is between 2.3 and 2.8 (1.3 stops). I'm not really sure what to do here. How do I open up 1.3 stops on camera that only has full stops?

Thanks -