Calorie burn acuracy?

dointhis4me

So I found a seemingly reliable formula online for calories burned of:
"For females, the formula is (0.074 x age + 0.4472 x heart rate - 0.05741 x weight - 20.4022) x time / 4.184."
"for males with the equation (0.2017 x age + 0.6309 x heart rate - 0.09036 x weight -- 55.0969) x time / 4.184."

According to this formula I burned about 1300 calories walking 120 mins at 3.5 mph for 6.6 miles. But when I put it into mfp, I get around 550.

What's going on here? Upon further research I see formulas that put me at around 700-800. I'm so confused, and now a little disheartened. When you guys workout on here with body bugs or hrm, where do you see your numbers in comparison to mfp?