I think a gain of five pounds in one month is probably about what you would want. The amount you are feeding is close to the amount I fed Riley at about that age and he was also very lean. I think it just depends on the dog and the activity levels. Poop is always a good indicator of feeding to little or to much.
"Never doubt that a small, group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has." - Margaret Mead