Trust them. Not me.
I must have screwed up. And I am doing corn sugar...
Look try the priming calulator yourself.
Okay, deep breath.... Priming calulater says: 15 gallons at 60 degrees and 2.9 volumeDextrose: 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 this priming calculator and this conversion calculator (which oddly doesn't have cups but 2 cups = 1 pint so ...) and stop listening to bozos like me on the internet.
Thx everyone. I ended up going with 600ml so might be a bit higher % than originally planned but oh well, all the merrier I'll be when I drink it