The best guide to how much food she needs is her weight
If she's "just right" then so is her food intake and there's no need to change it.
If she starts to get too thin as she grows (bearing in mind that you should be seeing the outline of ribs on a boxer), then that is cause to increase the food intake. And, of course, if she starts to look chubby, cut it back. A little too thin is far better than ever being a little too fat, especially with growing pups.