trainers advice please.

hypersensitiveb

On the cardio equipment after punching my info in age 37. Weight 161 the machines say for fat burn my heart rate should be aprox 119. I hit that walking fast with no hills with in like 5 min. To break a sweat I have to be on the arc trainer almost max speed on random hills on very high levels for at least 15 min. My heart rate is like 170-180 at that point. It says I'm burning like 300 cal in 30 min. Which seems very low too. I'm 5'5' if that helps any too.
So my questions are 1. Should I be exercising at the 119 heart rate or the 170-180? 2. Exercising at the high level aprox what do you think the real calorie burn would be? Thanks