My guys are grown , but they get a cup twice a day. One is 66 pounds and the other is around 44. Our girl is 48 lbs and not as active as our 44 pounder so she gets 7/8 cup twice a day. They are all maintains their healthy weights on this.
I was told iby my vet tech instructor to go by what's on the bag and know that the estimate on the bag tends to lean on the high side.