Power and HR workouts

First of all, I’m sorry for my English, it’s not my native language.
I use to train at home with Trainerroad with a powermeter trainer and on weekends I like to go outdoors to do my workouts, but I don’t have a powermeter in my bike so I use a HR sensor.
Now I’m doing Sweet Spot workouts.
So the question… Is it a good idea to use as a reference that 80-90% FTP and use it in the same way as HR zones ? That is, an 95%FTP effort is equivalent to a 95%LTHR?

Thank you so much

The problem with trying to do Sweet Spot intervals by HR is you have to take into account cardiac drift.
The longer the session the higher your heart rate will be for the same power.

If I look at a recent session (Eclipse) my 1st 20min Sweet Spot interval, averaged about 134bpm, however for my 3rd 20min Sweet Spot interval was closer 145bpm. That is quite a margin

Look at your own TR sessions and come up with a HR range that is appropriate for you

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Hi again,
Thanks for answer. That’s true, but it’s the same in indoors session. Of course in indoors you only have to wacht out the power but in every sweet spot while you’re keeping the demanded power output your HR is increasing everytime. Maybe the drift s not so dramatic as it was in outdoors but it still happens. I can’t keep 88-96%FTP each time with the same HR. I don’t know how to do.
Thanks in advance.