iPhones built in Pedometer & MFP's Step Tracker

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Hey everyone,

Something I have noticed since I got my iPhone 7 back in October, is that my calories from walking are significantly less than they used to be. Before the update of phones, if I were to walk 3 miles, I would be burning about 250 calories. But now if I walk 3 miles, MFP's step tracker says I burned 39-40 calories.

On Livestrong, I found a formula saying if I multiply my weight by .57 then divide that number by the steps I walked, it would give me my calories burned while walking.

So, my question is do I keep using MFP's step tracker for my calories? Or should I start using one of the built in exercises? Do I use the formula to find my calories burned?