If it's boiled then yes. If it's baked you can not use the 50g = 1pp rule. When you bake potatoes it evaporates the moisture from the potato (leaving it light and fluffy) so it'll _weigh_ lighter than it did when you put it in.
For example. I take a 200g uncooked potato. That is 200g = 4pp. If I bake this potato (wedges, chips, jacket potato, roast potato etc.) then it will come out weighing lighter than 200g (let's say it comes out at 150g for easiness). Even though it's lighter now, it's still a 4pp potato.
I always weigh before I cook my potatoes now.