There are a number of priming sugar calculators available on line. They will base the amount of sugar on style, volume and fermentation temperature. They are also built into most home brew software packages. They will typically recommend an amount by weight, which is a better measure than volume - so it is helpful to have a small digital scale (which is also helpful for weighing hops and DME).
All that being said, if one cup seems like a little too much, you can just use a little less. It'll still be good beer.