If you're just using a formula your heart rate zones could be quite far out.
For example, 220-age suggests my maximum HR is 178 and my threshold HR (85% of this) is 151, but in fact my max HR (determined empirically) is 188 and my threshold HR (based on an actual threshold test) is 162.
For example, 220-age suggests my maximum HR is 178 and my threshold HR (85% of this) is 151, but in fact my max HR (determined empirically) is 188 and my threshold HR (based on an actual threshold test) is 162.