Finding Outdoor ftp

I have my TrainerRoad zones set up as my power zones on the wahoo. However I know my outdoor ftp is more than my indoor ftp at my IF is always above 1 on my outside rides even when they’re over an hour. There is no road near me where I could do a solid 20min effort or even 2 8min efforts to find my outdoor ftp. Is there any other data that I could use to extrapolate my outdoor ftp.


Some alternate methods:

"the seven deadly sins…er, ways of determining your functional threshold power (roughly in order of increasing certainty):

  1. from inspection of a ride file.
  2. from power distribution profile from multiple rides.
  3. from blood lactate measurements (better or worse, depending on how it is done).
  4. based on normalized power from a hard ~1 h race.
  5. using critical power testing and analysis.
  6. from the power that you can routinely generate during long intervals done in training.
  7. from the average power during a ~1 h TT (the best predictor of performance is performance itself).

Note the key words "hard “, “routinely”, and “average” in methods 4, 6 and 7…”
Okay, so #7 is obviously the “Gold Standard”. What about the others?