Trial & error worked for me. I was near peak shape when I finally got a watch January this year, so I think that expedited the process but it took me about a month.
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@ChiefD said, if you're in appropriate condition put yourself through a sufferfest to try and figure out your max HR. I did an interval workout - 8x3 mins 'hard' to try and establish mine. No matter how hard I pressed it was very difficult to stay over 170 for any length of time. About 2 weeks later I did a 10K tempo that supported those findings - I was in the high 150's for a while but as I crept up to the mid 160's it got real uncomfortable, so I stayed below it until the final mile, when I pushed up to 170ish again. Sustaining in those mid 160's is what I did at that 10K race I got dq'd in just before the pandemic hit. It was uncomfortable throughout, but I was able to keep the pace and didn't have much of a kick left at the end so I think that was about right.
But where I think you will really benefit is in recovery. I very quickly found out that amidst training easy runs in the high 130's-low 140's were not actually easy. My priority workouts did not go well when the glue in between was comprised of that. Once I adjusted and kept my HR near 130 on those easy runs I noticed a sudden and drastic improvement in those priority workouts. There was more to it, but I went from tapping out early on a scheduled 16 mile run @ 7:39 pace (the last 3+ miles felt like death) to comfortably completing the same run @ 7:21 pace just three weeks later.
Now that I am months out of a training cycle I am not as restrictive with my HR on easy runs.
Since I'm not in as good of shape my HR has a tendency to fly up into the 170's when I'm not feeling like I'm working anywhere near as hard as I was in those winter workouts. I am sure to at least stay below 140 on those easy runs though. But whenever I enter a training before the training stage again I'll adjust back to what those winter/spring efforts said - the easy's around 130, the long's in the 140's, the LT's in the 150's, and the vO2's in the 160's.