Just Zones reads your workouts and heart-rate data from Apple Health to build your weekly time-in-zone charts. It only reads this data — it never writes anything back to Health, and it never uploads it anywhere; everything is computed on your device.
Just Zones needs Health access to show anything beyond sample data. If you denied it by mistake, go to Settings → Privacy & Security → Health → Just Zones on your iPhone and turn the categories back on.
Open Settings inside the app and go to Zone Setup. Just Zones estimates your zones using heart-rate reserve (Karvonen formula) with the zone bounds shown, and you can manually set your max and resting heart rate. You can also enter LTHR or age, and the app will always show which model produced the zones on screen.
Open Settings inside the app and tap Restore Purchases. Pro is a one-time purchase with Family Sharing enabled, so it can also unlock automatically for family members who share purchases.