you need to find the right level for you, though if you have five stone to lose, the top end will be fine. Losing weight requires you to learn where your sweet spot is (which reduces the lighter you get). If you eat too much you gain, if you eat too little, you stay the same or lose at a very slow rate, getting the level just right is the tricky bit.
If you follow SW plan religiously, sticking to free and superfree foods in the correct proportions, despite stuffing yourself it is very possible to be eating nowhere near enough that your body needs to function properly, causing it to slow down.
Eating your healthy extras and syns gives you a release in eating treats, but also pushes your intake up to that sweetspot. Medical experts say that no one should have a diet of less than a 1000 calories a day without medical supervision.
If you eat your healthy extras and your syns this gives you already 540, and thats before you have eaten your free and superfree.