Don't venture too far below 1200
You don't want to go below 1200 calories as a rule of thumb. Too few calories can cause your body to go into what is called "starvation" mode. On numerous occasions, I have had people tell me that they watch what they eat and exercise and they aren't losing weight. When I ask for specifics, 9 times out of 10, they tell me that they are eating about 900-1000 calories.
I tell them that now they have to eat to lose weight. I recommend that they bring their caloric intake up to 1300 - 1500 for a couple of weeks and then come back down to 1200. And generally, people will start losing weight again.