Here is a rough and ready spreadsheet for working out your HR Zones based on simple tests.
This gives your HR zones for running and cycling and swimming (if available to you).
Don’t faff around just do the 30 minute tests.
The test is: Run or cycle (or swim or row) FLAT OUT for 30 minutes. That speed/pace/power for the entire duration is as good as doing it for an hour. That is your functional threshold power or speed/pace.
HR is a bit different. Ignore the first 5 or 10 minutes as it takes your heart a while to get your heartrate up to speed. So the average HR for the last 25/20 minutes is your LTHR. Stick it in the spreadsheet.
Simple. Just do the test.
20 minute estimates also exist but you must follow a slightly more complicated and stressful protocol. OR get it done at a university lab for about £130.
The spreadsheet also gives you notes to say what is happening in each zone when you train in that zone. IE what good it will do you. It’s a simple but poorly-looking information-rich resource, please use it for what it is.