Treadmill Calorie Counts?

I walked at a 4.5 mile an hour pace on a 10 incline (it seems very steep in comparison to the others around me), and ran a 10 minute mile speed for short bursts in that time, with my heart rate staying between 150 and 170. At the end of the hour the treadmill said I had burned 635 calories. Does this seem accurate? I would love if it were the truth, but I feel like it might be under that.