According to a website I saw, gravity is measured in points per pound per gallon.
For dextrose, I THINK it was about 43 points. That means when 1 pound (454g) is dissolved in 1 gallon, it yields a gravity of 1.043.
If you made this 5 gallons, it would be 43/5 = 8.6. That means topping up another 4 gallons to make it 5 gallons would yield only 1.008.
If you prime at, say, 4.54 grams (to make it easy) per 500ml bottle, it means you would need 4.54 grams x 46 bottles to make 20884 grams (209)