I think your feeding instructions say two cups a day. I get the broke thing, been there done that but I think the Walmart food is about the same price and a bit better quality.
Your older dog grazing -I know you didn't ask so feel free to tell me to shut it, but it should be getting specified meals. As they age it becomes more important to monitor food intake versus condition. As they age weight gain or loss could signal serious issues and leaving food often indicates real problems.