How much should I be feeding a 90 pound, 9 year old dog?

There isn't any definite answer. A weight based table could lead you to continuing to over feed an overweight dog. Likely the package its chow comes in says 4-5 cups. What you need to do is evaluate its body condition and adjust its food and exercise to bring it into the ideal body condition. See LongLiveYourDog.com - Life Span Study - Rate Your Dog

Look on the package, also a 90 pound dog while large I have seen larger, but your dog is 9 years old, correct? If your dog is not overweight I would go by what the back of the dog food bag suggested. If your dog is overweight , again I do not know what breed we are referring to, I would go with a vet recommended diet type food to prolong your dogs life.