Accuracy of MFP and step counter?

fivefootfitness

I'm a college kid who walks around 3-5 miles a day just from class to class, and I also just started to training for a half marathon (currently only 3-5ish miles per session, on average). I know scooby's workshop BMR calculator says I burn about about 1600 calories (I'm extremely small), but when I look at the numbers formulated by my MFP step tracker on my phone, it usually estimates around 2100 calories burned per day. trying to lose weight gets tricky, because it's the difference between netting 1100 and being at the healthy 500 calories deficit, or being at a full thousand calorie daily reduction! Thoughts?