How to determine 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...