Yeah, with any of us, and any version of a DIY gizmo we choose, I bet we all could accept a certain difference between us and a nurse . . . just so it's consistent and predictable. If we accept the nurse's numbers are exactly right, and that she always reads about +10/+10 over whatever our contraption says, then we can do the math and decide if we like what we see at home on any given day or not.
I've seen consistent comment in mainstream media type advice that wrist readings are not as good as upper arm readings, but I don't know if the complaint is about the numbers being too different or too inconsistent. Guess the same principle exists, if you can compare your wrist device with your friendly nurse, then you can go from there.
Other alternative might be to buy a professional type cuff and sphygmo and stethoscope from a med supply house, dunno. There's never a @MikeRN around when I need one.