Dogs are greedy!
You should be able to feel the dogs ribs, without having to push through fat, but you shouldn't be able to see thyem with the naked eye.
Additionally, you should be able to see a waistline on the dog - a curve inwards just prior to the back legs.
I have a Cavalier too and feed 100g of Burns Chicken and Rice sialy along with a couple of busicuit treats here and there. With Cavaliers, its vital to keep their weight in check as a fit lean dog is likley to lessen the affect of the mitral heart disease that many Cavaliers get.
Don't be tempted by those big brown eyes!
A tin a day sounds about right.