Add a short (3-6 minute) maximal effort on a non-consecutive day within a week or so and you can compute critical power. I've foudn CP ~ FTP.

CP = [(long interval duration * long interval Pavg) - (short interval duration * short interval Pavg)] / (long int duration - short int duration)

OR

just use the Pavg you can typically sustain for long (20+ min) intervals in training