A gallon of water is
8lbs. Therefore a five gallon bucket of water should weigh
40lbs; obviously zero out your scale for the weight of the bucket itself or subtract it from the total you get. This is the most certain weigh IMO to check your measurements since measuring containers can all be off; then we are faced with using a scale we know is accurate. ;P
Apprx 9.5oz. per gl. isn't going to change the world with any car shampoo IMO. It's the true chemicals used in detailing that need to more carefully diluted IMO/E. That said, try checking the accuracy of your measurements/ratios on your spray bottles. That is where I usually measure my solutions in measuring cups, not by the side of the bottle unless I have measured the bottle for accuracy beforehand.