Custom Heart Rate

I would appear that the 0 value, as well as the maxHR is not editable; at least from my attempt.

The zone value is based on your maxHR, while the % would be against your LTHR.
I would use the 80/20 rule, and set up your 5 zone setup using those 5 fields labelled 2-6.
You can rename them as I have in this example:
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