My max HR is 10-15 bpm higher than what I hit during an FTP test. I actually found my max during a Zwift race. I knew my resting HR after months of measuring heart rate and blood pressure in the morning. With those two values I used the “Karvonen modified” calculation from this page to get my zones. They seem quite accurate for me, and match up well with an earlier indoor TT I’ve done. Joe Friel also recommends the 30 minute TT method to find FTHR, but I just can’t be bothered doing one 
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