Know your data. Know your Zones.
To train effectively and optimise your fitness you must know your current level of fitness (your key personal data & zones) and have regular 1:1 progress checks.
People are different. In fact, only 20% of people have a max heart rate that is functionally close to the 220-Age = Max HR calculation, which is what most people use to determine heart rate zones these days.
This means for a large majority of people setting zones of 50%-100% off of that formula does not result in zones that are meaningful or correct.
Knowing about your body is important, and just being aware of what your maximum heart rate should be in a controlled environment can help you monitor your exercise. However, for those who want to exercise intelligently and be healthy through exercise, setting and monitoring personal heart rate zones is essential practice.
When you are trying to optimise your fitness level, custom zones become not just important, but critical. When you are training for optimal performance, it is widely agreed that the only thing better than heart rate training is training with power sensors.
If you don’t know your personal zones, training for optimising performance using estimated zones is simply a crap shoot. Without accurate zones, it’s like training in the dark. Perceived effort is fine for some, but nothing beats working out with heart rate & power.