Is she actually overweight though?
The breed standard is a guide for what the breed should look like, but many dogs are larger than the breed standard, and a heavier weight is absolutely correct for those dogs given their frames. For many on this board, having their dog down to the top of the breed standard -- 18lbs -- would be an emaciated dog and very unhealthy!
That said, most of us have the reverse problem of too-heavy dogs and we tend to not see how heavy they have gotten. At a healthy weight you will be able to see a waist on the dog when viewed from above and easily feel the ribs (without having them sticking out). Some heavy coated dogs will look heavier than they are from above so the best time to check a dog's waist is when it is having a bath and is wet.
You can see dogs at the right weight here:
http://roycroftcavaliers.com/manualfeeding.htm
From that page, this is what a dog in GOOD weight should look like (I bet it is considerably slimmer than many of our dogs!):
As Laura stresses on that page, how much a dog needs depends entirely on metabolism, exercise etc and varies widely. I have two that eat only 1/3rd a cup of food daily and are quite active. I have another that easily eats 3/4th to a full cup daily and burns it off. That's why you cannot trust what it recommends on food bags. All foods have different calorie values too.
Good for you for planning on her health!
I cannot stress enough how crucial it is to keep dogs in good slim weight. Recent studies show you cut up to a FOURTH -- many years! -- off the life of your dog by having it overweight, and that doesn't mean obese, it simply means *overweight*. In this breed it also puts serious extra stress on the heart to have to supply blood to more body weight and to have narrowed blood vessels due to fat -- and that definitely shortens the dog's life and hastens death from MVD.
So on every single count -- all of us need to keep our dogs slim, we need to have a very light hand with treats and meals, we need to get them the exercise they need. It is just so important -- who wants to lose their dog years before their possible lifespan?
In the library section there's info on pudgy pets, healthy treats, etc but the link above has much of the same info.