There's a good journal article that compares different perspectives and measurements for zones, that might help (if you're interested in the science!): https://journals.humankinetics.com/view/journals/ijspp/aop/a...

Threshold test might help you dial it in, and it doesn't have to be in a lab (though that would be most accurate) -- you could do a workout effort to estimate your max sustainable heart rate for an hour, then calculate zones based on that threshold HR (i.e. my Garmin calculates based on % LTHR, where zone2 ends up being 83-89% of LTHR).

It's also not an exact science - it's a spectrum and shifts day to day depending on lots of conditions!