I think the 220- minus your age is a very rough estimate, and max heart rate can depend on a lot of factors and will change as you get fitter.
I would invest in decent heart rate monitor to start with an actually calculate your real base heart rate (taken first thing in the morning whilst still in bed, and done over a few mornings to get a true average), then calculate your true maximum, i was always told to calculate it by:
– 10 minutes warm-up
– the 5 minutes as hard as you can manage.
– then 5 minutes back down to Zone 1
– then go as hard as you can for 20 minutes really trying to go flat out.
Somewhere in that 20 minutes if you’re going hard enough, you should reach your max heart rate
From that you should be able to calculate your training zones.
If you have concerns, then it would be worth addressing them with GP. But heart rate monitor is good for looking whether you are over doing it, if you are and not recovered well enough , then your heart rate tends to sit too high when you test it first thing in the morning