Good rule of thumb to go. Take there racing weight add 5% to that. Take that number for the weight you want them to be and divided that number by 15 or 20 and that will give you the cups per day. If you look on your dog food bag that might be the cups per day the manufacture recommends. Have heard this formula form a lot of vets and a lot of articles read on this subject. Another good rule of thumb if your getting a lot out of the south end so to speak your either feeding to much or its not a quaility dog food. A lot of poop means what your seeing is the filler the manufacture puts in there food. The key also is plenty of exercise for them.
Woolfy