Never base zones off what you think your MHR is. You'll be wrong. Instead, base them off your lactate threshold. That's determinable by testing. There's a sticky on that at the top of the Training and Nutrition forum. You can also use the CTS field test:
https://trainright.com/cts-field-tes...-calculations/
A really simple way to get close, like tomorrow, is to warm up really well, maybe an hour, then start a good long climb. Keep ramping up the effort until you start to pant. Note HR, then gradually back it off until you return to deep breathing. Note HR. Ramp it back up and down a couple more times, noting the HR when you are deep breathing and before starting to pant. That's probably close enough to your lactate threshold to use, right there. That doesn't work for everyone because is assumes your efforts are not limited by asthma or lung volume, but few people have that problem.
I have also have a good method for trying to find my MHR. I have a 4000' climb nearby which ends at over 6000'. I ride that climb hard, then take it up to near lactate threshold for 45', then over LT, panting hard, for the final 10', then sprint into the parking lot at the top. That produces a HR I haven't seen all year. Might be my max. Maybe. I've ridden that climb so many times that I know exactly where those times are.