1 tablet for five gallons so I agree that 1/4 would do it. Yes to the fridge also. Try to cold crash it a a lower temp than serving, close to freezing if possible. Your fridge should be in that range anyway.
I also put whirfloc in everything except something like a Hefe Weizen that should be cloudy. If you hold your pint of Porter up to the light to check out the color, you will want it not to look cloudy, just dark.