What would you consider zone 3? I track my walks with my apple watch and am generally at 125-135 for the majority of the hour I am out.
Agree with
@BigLou one post above. I would make it simple and call it 75% plus or minus a handful of percent percent, if you want to do a formula made for the general public and has no individual variation built into it.
There is the age-based maximum heart rate that is a blanket statement made 30 or more years ago and it hasn't changed.
Then there is your actual maximum heart rate. An individual's maximum heart rate is based on current fitness, predisposed genetic factors, actual physical size of one's heart and probably some other stuff that I've forgotten over the years.
Years ago I did a maximum heart rate test, and that simply involves pushing yourself till you think you're going to pass out and see what your heart rate is with a heart rate monitor. This takes into account current fitness level and it varies as fitness level varies.
The highest I ever saw in the prime of my adult fitness was 182 beats per minute. At that time my age based maximum should have been 190. This was during training for a marathon, Pikes Peak ascent, Xterra triathlon.
For comparison, my 2-year younger brother was training for the same things. His maximum heart rate pushing himself to the limit was 205.
Regardless of the formula, two guys within two years of each other from the same parents, varied by 23 beats a minute. The formula is nothing more than that, something created as a general guideline but it may be misleading.
Point being, do the best you can to find out what your max heart rate is for you as an individual. Do calculations from that if you want to fine tune it for yourself.