Today, I brought my DIGI DMC-688 powder / quarter scale to weigh the powder to get the exact amount of powder for a 5 gallon bucket.
First, I zeroed out the plastic bag. Then I weighed about 5 pounds which was 12 1/4 cups. So to make 5 gallons of JBS Industries Hydro White Powered Tire & Engine Cleaner just use 12 1/4 cups or my previous guess of 12 cups.