You can base it the total amount of beer rather than the amount per bottle. http://www.northernbrewer.com/priming-sugar-calculator/ This helped me alot you pick your style or wanted carbonation level, your total volume, and temp at time of bottling and it gives you multiple options for the sugars. then boil that with roughly 2 cups of water, add it to the total beer volume mix gently and bottle.
For 12oz bottles, I add 2.5 grams per bottle. If you can find them, Domino Brand has smaller sugar cubes that are 2.5 grams. Place one cube in each bottle.