calories you ate less exercise calories burned is a completely meaningless number. it compares your intake to part of your output, which doesn't really get you anywhere. it's kind of like if you wanted to calculate the mileage you're getting per tank on your car, and comparing everything you put into the tank to fill it up versus a small errand run that was the only mileage number you bothered to take.
your bmr [the calories you would burn everyday pretty much being in a coma/lying in bed and not getting up] is the big player in your expenditures. you add to that your daily activities [generally about .2 of your bmr] to account for basic things like getting out of bed, brushing your teeth, showering, and all those movements that you make that aren't exercise. then you add your exercise calories to that to get your total burn for the day.
once you have your total burn for the day, then you subtract out your calories eaten to get your deficit for the day.
if you want to know more about how to actually do the math on this you can check out www.sparkpeople.com/resource/calorie_calcu
and your calorie minimums are to get basic nutrients in you. you don't need to randomly subtract out other numbers, just make sure you are hitting 1200 with a variety of foods, and you're likely hitting where you need to on the nutrients.