bodymedia fit!

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So I have been eating (at what I calculated to be) a calorie deficit of about 200-300 calories. I bought a bodymedia fit because I wanted to see if I was burning as much as I thought I would. I figured I would be burning about 2200-2500 a day (tops). I've been using it for a week, and realised on average I burn about 500-700 more calories than I thought I did. Has anyone else severely over or underestimated their activity levels?! What did you do?