Another method for calculating your training zone is the Karvonen method. This method takes into account your resting heart rate as well as your age predicted maximum heart rate. Since the Karvonen method takes into account the heart rate range between resting and maximum, it is often preferred by health care providers. It is for fit and healthy individuals, who have trained for some time.
In order to use this equation, you must know your resting heart rate. Take your pulse 3 mornings in a row, BEFORE getting out of bed and doing anything. Calculate the average and use the following equation.

(220 – age) – resting heart rate = HRR (heart rate reserve)
HRR x .60 + Resting heart rate = Low end of training zone
HRR x .80 + resting heart rate = high end of training zone

Using myself as an example, you can see the difference in using the straight line method and the Karvonen method.
220 – 40 = 180 X .65 = 117 (Fat burning target heart rate)
220 – 40 = 180 – 55 (rhr) X .65 + 55 = 136

I hope this helps.