Hurrah! A lab pup at a healthy weight! You are to be congratulated. I wish every lab pup that came into the clinic was as fit. Labradors are notoriously difficult to keep at ideal body condition for two reasons. First, they often will eat anything and everything and not know when to quit and second, many owners don't realize that the stereotypical square lab is actually overweight.
As a general guideline, with any dog you should be able to see a modest waist from both the side and the top. Also, if you lightly rest your fingers behind the shoulders on the sides of the ribs, you should be able to press just enough to dimple the skin and feel ribs. They shouldn't feel like your knuckles, but you shouldn't have to hunt for them either.
Ok, there's my soapbox for the day. :)
When Ruby arrived, she was a little butterball.
The breeder fosters her in-whelp bitches out to friends so that the pups grow up underfoot in a home with kids and commotion. She said the only problem with the lady who fosters Ruby's mom is that she overfeeds them. Otherwise it's a perfect situation - 40 fenced acres, pups get to play outside with the mom in a puppy paddock - My Shelley was also a butterball when she arrived, but that was because she was Boss Dog in a litter of nine and got more than her fair share of the food.
My vet competes her own dogs in agility and is a stickler for keeping them at the proper weight. We've actually had to adjust Ruby's food UP - she burns a lot of calories just kangarooing around.
The Labrador standard says there should be little or no tuck. I showed Logan as fat as I would ever want to get him, and he still wasn't nearly as fat as the other labs. They look like sausages with legs, but the judges keep rewarding it.