I have high blood pressure, managed with a low dose of medication. I've had plenty of advice from my PCP and other actual doctors on how to take blood pressure - how to sit, how long to sit still, how to position your arm, etc.
It is remarkable how many medically-adjacent professionals are bad at this. My dentist starts my sessions with a blood pressure reading - theoretically in case they have to numb me or something, probably because it's billable or whatever. What always seems to happen is that I get in for my appointment (driving, which can be stressful even if I'm not running late), they take me to the chair within 2 minutes max, immediately put some sort of wrist cuff on me, hold my arm at an angle that is not what most actual doctors have recommended to me and take a reading that is often high enough to surprise both the hygenist and myself.
The thing is that I have two sufficiently-calibrated Omron units, one at home and one at the office. I take my blood pressure often enough to know what it usually is (and my real doctor takes it at least twice a year). That is to say that no, it's not 200/160 or whatever nonsense the dentist thinks it is and you'd think that with a very small amount of reading they'd know better.