I got a total of 3.3 pounds so I'm not far off.It's a lot easier in metric! Density of water is 1gm/cc or 1000g/1000cc (liter). Shop manual says to use 10% citric acid. So you add 100gm per 1000gm of water (1 liter). (100gm=3.527 oz)
Capacity of cooling system is 15L? So, you would need 1500gm (1.5 Kg) of citric acid powder in 15 liters of water. (1500gm is about 53 oz.) So your 40oz would be a bit weak, but probably OK!
Best to stay away from fluid oz - those 40 ozs only apply to beverages