I think this is good too as a means of triangulating when you are going too hard. Especially as one becomes more fit and the differences between zones become larger (i.e. high threshold). It also lets you easily do the work without power (as this will and should happen due to device failure, different bikes etc). For sweet spot, something around ~88%MHR is one I use.

As you have suggested learning to feel when Threshold is becoming a TT (i.e a Vo2 interval or >105%FTP ) becomes more important. For sweet spot (like tempo) we want to make sure you maximize TIME and INTENSITY so that you get a lot of work in but also can come back soon. If you start dipping into the higher ranges then you would expect longer recovery and less work in the longer term.

It is this time in zone both in the session and over many weeks that is often missed. It is easy to ‘feel good and push it’ today and then not do anything for a week (or more!) due to fatigue and illness.