If you only eating 800, and burning 250 through exercise then you are only taking in 550 which is nowhere near enough. You need to NET 1200 so technically you'd need to eat 1450 (if you burned 250 through exercise) to net the 1200. It's really confusing but it's easy once you know how.
I have a fitbit and this helps me meet my goals. On a day where I've reached my exercise goal and steps, it'll tell me to roughly eat 2000 calories and it will tell me I have burned 2500ish. I have set it to be at a 500 calorie deficit. If I wanted to maintain I'd take in the full 2500. Seems a lot, but if you're very active it makes sense.
It seems to me you have put your activity level to sedentary? Is that correct? 1200 seems so low. When I used mfp it told me to take in 1450 at sedentary (as I was at the time) and then add exercise cals.
I've lost 4lb since getting my fitbit (which was coming up to 2 weeks ago). I find it's so much more accurate as it adjusts your calories depending on your activity. No need to work it out for yourself.