Calories Burned

Not sure if this is the best place to ask, but figured there's a lot of highly qualified people in here. How accurate are the calories burned readers on tredmills and stairmasters?

How many calories should I be burning on the tredmill...speed 3, incline 12, 30 minutes? How about stairmaster for 30 minutes, levels 5-8?

35F, 116 pounds if that matters. I always do cardio after lifting. TIA!!