2/3 is only a guide isn't it? If you are making up the recipe for 4 I'm sure you can share round any rice that you don't need, or rice keeps in the fridge for a few days which isn't a bad thing to have on standby as a free food
Looking at the recipe I would say 50-60g of dry rice per portion to make up 2/3. Once cooked it will soak up water and weight about 125g ish which is how much salmon is in a portion.