It's always better to measure priming sugar (or any dry ingredient for that matter) by weight instead of volume. Recently Alton Brown did a demo where he had two identical sized containers of flour one where the flour appeared to fill more of the container then the other. When he weighed them the larger appearing volume of flour actually weighed the same as the container that appeared to have less. Dry material will settle. So measuring by volume is faulty.
So get a cheap digital scale and weigh out your sugar instead. You can get one for 20 bucks at most big box stores.
That being the case, the "standard" for producing between 2 and 2.5 volumes of co2 (the average carb level for most beers) is 1 ounce/gallon of beer.