My heart rate is all over the place so I don't use it as a metric for anything. Due to a thyroid problem my resting HR can vary between 60-90. Last year before thyroid cancer I was able to max out around 160-170 bpm on several indoor trainer sessions, probably about right for my age (61). This year I'm having trouble getting it above 150.

I can't even judge by my average speeds and times over familiar 20-30 mile training circuits. My speeds now are the same as last year before I got sick. But I'm wiped out for a day or so afterward, unlike last year.

Until my thyroid level is settled by meds I'm not paying much attention to heart rate. I just go by felt exertion and how I feel the next day.

A friend suggested a gym with a retired pro cyclist coach/trainer who does FTP tests, training with power, etc. But for me right now that's a waste of time and money. I'm pretty sure the guesstimation calculators from Strava and the Elevate extension are pretty accurate -- my average power over distance is around 160 watts, so it's a waste of money for me to buy a power meter or worry about it until my metabolism is stabilized.