Okay, deep breath.... Priming calulater says: 15 gallons at 60 degrees and 2.9 volume
Dextrose: 451.08 grams = 15.91 oz. = 2.26 cups. which is 534.7 mL.
But let's double check by looking up the weight of dextrose on wikipedia... hold on a minute.
... can't find it...
Use and (which oddly doesn't have cups but 2 cups = 1 pint so ...) and stop listening to bozos like me on the internet.