FWIW my recommendation is to follow Joe Friel’s The Cyclist Training Bible and use % LTHR
My LTHR hasn’t changed in years. Its possible to average above LTHR, say 165bpm on a hard 20-30 min effort, but that is due to anaerobic contributions. On longer steady state efforts at FTP, my HR is usually within 2bpm of 161bpm. Other sub-FTP zones do not always nicely fall into zones. Hope that helps.