When looking at the dog from the side, I like to see an obvious tummy tuck. And looking at them from the top, you should be able to see a "waist". You should be able to feel the dog's ribs when lightly touching them, but shouldn't neccesarily be able to see them.

In my opinion, it's MUCH better for the dog to be slightly underweight than to be overweight.

I wouldn't really recommend feeding what the food tells you to feed. Often times they way overestimate how much you should feed. If you're feeding the dog twice a day, you can cut back the amount. If the dog looks too skinny, up the portions a little. Eventually you'll find out how much your dog needs.

Overweight dogs is a big pet peeve of mine. In fact, my family tends to feed my dog much more than I would feed him and as a result he's the heaviest he's ever been. I'll get that 5lbs off of him this summer, only to have him regain it when living with my parents during the school year again.