Start with 220
Subtract your age by 220
Multiply your maximum heart rate by .65 and .80 to find your target heart range.
This method is more effective because it takes into account of your Resting Heart Rate (RHR) which is an indication of cardiovascular fitness.
Example; For a 15 year old, you would do 220 - 15 = 205
Usually, people who undergo regular vigorous exercise generally have a reduced resting heart rate. The formula (RHR) adjusts target heart rate in response to cardiovascular adaption.
The formula is; (220 - age - RHR) (Intensity) + RHR = Target HR
Your age = (Lets say, 52)
Resting Heart Rate = 10
Exercise Intensity (Low end) = 10
Exercise Intensity (High end) = 10