Galadriel, the 2/3 cup you are thinking about may be the "mashed, frozen, prepared" type, which is listed in the Complete Food Guide as 2 pts. My leader said that the points were based on an average taken from sampling typical brands and various restaurants.
If you make mashed potatoes at home, you can get a little more for your points, as 10 oz of raw potato is just 3 pts. If you cook it in chicken broth (0 pts) and spray butter (0 pts if not overused), fresh cracked pepper and parsley, using the broth instead of milk, you can get a huge serving of it for 3 pts. For an extra point, you can stir in 3 T of light sour cream. (Can you tell, I really like mashed potatoes?) Yukon gold makes the best mashed potatoes, IMHO.
256/156/155 (pg144) Lifetime 7/01