Take your pulse (at wrist or neck is best) before you get out of bed. Count the # of beats in a minute. Then, knowing this as your true resting heart rate, you can find out what intensity you need to be working at during cardio in order to reach your goals. If you perform a 3 minute step test, take your pulse immediately after, you can use these 2 pulse #'s to find your Heart Rate Reserve, and you can use this to find a target heart rate range that is subjective to you.
Maximum Heart Rate assumes everyone of a certain age has the same resting heart rate, and this is not fact. By using Heart Rate Reserve, you can find the target heart rate range to meet your specific goals.