I raced a 40km TT two weeks ago and 20km TT last Saturday. Rather than do a seperate 20 minute test, I just usually use the last 20 minutes from my many regular 20km TTs.* For me, comparing the 40km TT and last 20 minutes of the 20km TT confirmed the *0.95 as accurate for me.

*To be added to Alex's deadly sins if not already there