Typical bottle carbonation consumes 2-3 gravity points of sugar. So when you get to back sweetening before pasteurization, add an additional few points for the bubbles.
Let's say you want to add another 5 gravity points for sweetness. Dextrose is 42 ppg, so
5 points / 42 points*gal/lb * 5.5 gal *454 grams/lb = 297 grams