I have a different take on this. Essentially because I’m dong the trial of Polarized periodisation instead of Conventional or Sweet Spot.
Staying in the right zone: On TP podcast 309 this is also discussed more generally, ie, not Polarised specifically, rather doing workouts outside, one conclusion being don’t get too hung up on specific FTP, they are targets, and in fact RPE, the alternative to power for outside, offers some benefits. That the purpose of the computer tools is really to help you become more in tune with your RPE, awareness of your own body at different levels of exertion.
So one way of helping identifying these zones is the ‘Talk Test’. May not seem high tech enough for many but apparently is still a great method, and much more closely related with RPE and just as important the Ventilatory Thresholds associated with Polarised training. The 3 zone model, is basically: Low Intensity Zone 1 is below VT1, Medium Zone 2 is between VT1 and VT2, and high Zone is above VT2. The talk test, below VT1 you should be comfortably capable of saying a large part of the Alphabet before the desire to breath kicks in with the top of that zone being about A-J or the same as your Mobile number; between VT1 and VT2 by the top of the zone you may only be able to manage the first part of your mobile or A-D; beyond VT2, it start to become difficult to talk at all, from say a yes/no, teenage grunt and the like. An important aspect is having to say it out loud, this isn’t self talk! I’ve been using this on my outside rides and find it useful and closely related to the RPE and FTP targets. Some additional background explains the bodies response at VT1 and 2. Below VT1 the body just has the need to Inspire O2; beyond VT1 Expiration of CO2 becomes significant and by the top of Zone 2 you are increasingly aware of blowing out CO2; above VT2 it’s your primary focus, eventually leading to hyperventilation.