How thin should a dog be to be in good shape?

[VeryStressedMum]

I have a lab and the vet once said she's in good shape because when you run your hand over her you can feel her ribs, obviously not protruding ribs but she should go in slightly at the waist (or what ever you call the stomach bit on a dog), and you should be able to feel the ribs, but I always think it looks too skinny and think I should feed her more....she's active and healthy but is always on the scrounge for food (which she would be like anyway even if she weighed 20 stone).
So just wondering what shape is the best shape, should she go in at the stomach or should she be a bit rounder? probably a strange question but I think I want to feed her up a bit but don't want to make her unhealthy.

My 2yo lab is a working type and very lean. Most people comment that he is too thin for a Labrador. My vet says he is perfect and you should be able to see at least two ribs each side.
He is full of energy, bouncing , leaping and running everywhere. He wouldn't be like that if he wasn't fed enough.
It's sad to see dogs, especially young labs, that are overweight and plodding around on a walk.

Oops.

Ribs is fine - there's a difference between seeing the rib outline and seeing ribs, you will see that if you have a waist on a perfect bcs 3.
Feel the ribs, feel the spine and see a distinct waist from the side and above.

You are used to being surrounded by fat dogs probably.
Good muscule will highlight contours as well. Many dogs have about half a muscle between them which lets it all hang out so to speak.

Thank you, I feel better now that I'm not starving her! She's quite well muscled I think you can see the muscle. Hopefully she's healthy enough to live for a good few years yet!!