Heart rate and calories burnt question

  • Ok so i hit the gym!
    BUT I always wonder, how do I know whats a good heart rate to have while doing cardio?
    I know there is a way to measure if the heart rate you have is good for the kind of exercise you're doing..I just dont know how it's measured.. any ideas?
    Also, I burn about 400 - 430 cals/60 minutes. I do about 7 - 8 miles of walking on the treadmill every alternate day and a half of that on the rest of the days coz I add in strength training

    whats a good or rather decent calorie burn/hour for cardio?