you need to try to divide your plate up visually. It is hard to say if something is definately a 1/3 superfree without seeing the size of your other portions.
But for instance, if you had a a piece of chicken the size of a fist and a jacket potato the size of fist, then of course your portion of superfree should also be of equal size. Of course you may not always have carbs and proteins together, you may just have carbs or just proteins, but again the same principle would apply, depending on the size of that portion, your superfree should be a 1/3 of your entire meal.
so for instance, imagine this as your plate:
it doesn't really matter what free foods you have in 2/3's of the plate as long as one of the 1/3's is made up of superfree.
Of course when making things like spaghetti bolognese or chilli etc it is harder to judge the 1/3 superfree, because you are mixing everything together. But for things like that, I generally make sure I am adding at least double the amount of vegetables to how much meat I am using, so that once it is served up with a carb of my choice, it is pretty much guarenteed to include a 1/3 superfree.
Hope that makes sense.