You can also use any number of target heart rate calculators to get your heart rate
zone, but many of them do not incorporate your resting heart rate
(which makes it a bit more accurate).
Below is an example of the Karvonen formula for a 23 year old person with a resting heart rate of 65 beats per minute (to get your resting heart rate, take your pulse for one full minute.)
220 - 23 (age) = 197
197 - 65 (resting heart rate) = 132
132 65% (low end of heart rate) OR 85% (high end) = 85.