Had to do some conversions as I wasn't familer with stone's 1 stone = 14 lb x 3500 calories = 49000 calories divided that by the days you want to lose it over and you get 49000/9x7) = 777 calorie deficit per day. You need roughly that calorie deficit per day to acheive your goal. As for how fitbit estimates the calories you can consume it will depend on if you have it set to personalized, or sedentary mode (this can be changed on the food logging page online using the litter gear symbol). I use mine as sedentary which assume you haven't done any activity for the day and will add your BMR + activity calories as you earn them throughout the day. The other setting of personalized will use your average activity level over several days to give you an approximiation of your total calories for the day at the beginning. If you are using this method do not eat 1800 calories for breakfast to be in the zone.
It is good to be in the zone near the end of the day although if you go a little over or under with earlier meals dont worry about it. Dont force yourself to stack small amount constantly to keep it there. I personally like to aim a little under the "in the zone mark" as I am trying to lose around 3lbs a week which isn't a setting allowed by fitbit, not unreasonable for me weighing 280 lb's.
