How much food is too much food?

Our pup is now 5 months. She's a rescue found with her mum who is a German Shorthaired Pointer. We've had her for almost two months now and despite putting on some weight, she is still thin.

I understand GSPs are a thin, athletic breed and are naturally narrow around the hips, but her ribs are still quite visible without feeling them. We are feeding her a grain free complete puppy food and following the recommendations of 365 grams per day (split into 3 meals) which is the high end of the guide for her expected full adult weight.

Our vet she is healthy, however her brother who was adopted by someone we are in contact with was told he is too thin and underweight by their vet (same weight, both 8.5kg).

If we increase her food per day above the recommended amount on the bag, say from 365g up to 450g (per day), is this responsible as long as we are monitoring her weight and ensuring she doesn't get too fat?