This is known as the Karvonen Formula (not to be confused with the Kevorkian formula)


220 - (your age) = Max Heart Rate (MaxHR)
MaxHR - (resting heart rate) = Heart Rate Reserve (HRR)
HRR x 85% = training range %
training range % + RHR = high end of your THR zone

This example shows the high end of a THR for a 35 year old person with a resting heart rate of 60 bpm:

220 - 35 = 185
185 - 60 = 125
125 x 85%= 106.25
106.25 + 60 = 166 beats per minute