It takes some experimenting. I'd start with the 1800 and see where that gets you. Most women are best off in the 1400-1800 calorie range, I'd say.

Because of my height and body type, I can get away with doing 1100-1200 (I try not to go below 1200) and feel completely satisfied while getting all of my nutrients. I also fluctuate it- some days I go to 1300 or higher.

You just have to kind of experiment. It'll become natural to you!