will i lose comsuming 1200? how much?

Too few calories will knock you into starvation mode, and your body will hang onto every calorie it gets. BTW, you're at normal weight: http://www.nhlbisupport.com/bmi/

I suspect you don't need to lose but, rather, would like to firm up. If so, exercise -- and get enough to eat in order to fuel that.

You need to exercize. It takes 3,500 calories to burn off one pound of fat. Walking two miles an hour will burn off 100-150 calories, riding a stationary bike at about 11-12 miles an hour with moderate tension will burn off about 500 calories in a hour of pedalling.

The only way to lose weight is to burn off more calories than you take in. Remember (and especially when you put that tasty Snickers bar to your lips), it takes 3,500 calories to burn off one pound of fat. That is equivalent to six-seven hours pedalling a bike at a fairly rapid rate.

Your idea might allow you to lose a few pounds, but likely you will gain it back because doing what you stated will result in the body assuming that you are fasting, and the body begins to alter its metabolism to account for the loss of calorie intake, making the body's metabolism more efficient and requiring less caloric content to perform its tasks. The problem arises when you break your fast with the body metabolism being more efficient, the excess caloric content becomes fat. Dieting without exercise is futile since dieting alone; i.e., restricting caloric content simply drives the body into a metabolic state that burns less "fuel" or fat.