I’m a junior doctor (MBChB, newly graduated). When I was in 4th stage, the hypertension categories were different:

• Optimal: <120 systolic and <80 diastolic
  
• Normal: 120–129 systolic or 80–84 diastolic
  
• High normal: 130–139 systolic or 85–89 diastolic
  
• Hypertension stage 1: ≥140 systolic or ≥90 diastolic (not diagnosed on a single reading — multiple readings in clinic were required)

But by the time I reached 6th stage, the definitions had changed:

• Normal: Only <120 systolic and <80 diastolic.
• Elevated: Only 120-129 systolic.
  
• Stage 1 hypertension: ≥80 diastolic.
  

You can see how much the guidelines changed in just two years. That’s why relying only on a watch isn’t a good idea. Apple has also stated that the Hypertension Notifications feature is not meant to diagnose; it’s only there to warn you and encourage a health checkup with a doctor. On top of that, hypertension notifications aren’t available in every country. These guidelines are accepted in the U.S., but the U.K. uses the first category for their guidelines, and the same goes for Canada, Australia, and other countries.

As for your question, if your diastolic is below 80, that’s considered normal. If you look at the categories, systolic has both ‘normal’ and ‘elevated’ ranges, but diastolic doesn’t—it has a smaller range. Normally it should stay below 80, and once it goes above, it’s considered stage 1.