my cat is a bit too chubby

Catherine, Usually a good way to tell is that you can feel their ribs fairly easily (without alot of fat in the way) but you shouldn't be able to see the ribs that would be to thin.

The chart listed on the Purina site http://www.placervillevet.com/feline%20body%20condition.htm shows you what your cat should look like. It's hard to tell a 'weight' because it's very dependent on the structure and breed, etc. but most vets go by a visual chart such as this. Good luck getting her weight down. Make sure she gets a check up if the weight gain was sudden, it could be a sign of illness.