It's amazing what part of 128 cubic ft. people don't understand. A cord is only a cord when it's cut into 4' lengths x 8' x 4'. What happens to it once it is cut and split depends on several factors. Regardless it's best to be fair and give a full measure+. I always liked 'bin volume', ie loose thrown wood, meaning 12" wood ocupies 160 cubic feet while 16" wood ocupies 180 cubic feet, accurate to +- 5%.
It's not so much that we are splitting hairs, it's just that math tells us that a 16" cord will stretch out 24' but try to fit that within 128cu.ft. It just wont happen.
Regardless, keep the customer happy, give a full measure and you'll have them for a long as your cutting wood.
John