An Imperial Gallon is 160 Imperial Fluid Ounces, which means an English weighs 10 lbs. A U.S. Gallon contains 128 slightly larger fluid ounces, and is defined by cubic volume rather than weight in water. It is the old Queen Anne Wine Gallon that was adopted by American colonists for all liquid measures, not just wine. Anyway the U.S. Gallon is 231 cubic inches. Now, in terms of weight, a U.S. Gallon is about 8 1/3 lbs. Therefore, since a U.S Quart is 1/4 of a U.S. Gallon, 32 fl. oz., 32 U.S. fl. oz. x ~1.04 U.S. fl. oz./imperial ounce, you have about 33 fl. oz. 2 fl. dr. (33.28) Imperial Ounces.
Honestly, if you just remember to use a 128 gallon instead of a 160 ounce gallon you'll be fine. 33.28 compared to 32 is a negligable difference.
Remember too, all of those touting metrics, that one, metrics have never taken man to the moon, but customary units have, nine times, two most measuring containers commonly used in darkrooms only go down to 25 or maybe 10-mL increments, so they aren't any more accurate. Further you can divide U.S. increments in halves to 1/2048 of a gallon (not so much for the Brits, but they can also divide their gallon in thirds)
U.S. liquid measurement is actually quite a nice, easy system.
1 U.S. gallon (~3.76 L)
equals 2 pottles (half gallons)
equals 4 quarts
equals 8 pints
equals 16 cups (although a cup of coffee is usually 6 U.S. fl. oz.)
equals 32 gills
equals 64 jacks
equals 128 fluid ounces (85 1/3 ponies 1 1/2 oz.)
equals 256 tablespoons (768 teaspoons, 1/3 tbsp ea.)
equals 1024 fluid drams
equals 61440 drops minims (1/60 of a fl. dram)
Also, degrees Fahrenheit, being smaller are inherently more accurate for temperature measurement, since they are 5/9 of a degree Celsius. Miles, feet, and inches also allow for nice halving several times over, and in addition allow easy division by thirds. When I drive 90 MPH down the interstate, I'm going 132 feet per second. Everyone else around me going the speed limit is going 88 feet per second. Do that with kph and mps.
Hope this helps.