Calculating LT Power (Power zones)

Hi!

The 80/20 Triathlon book suggests that we find our rFTP using the following method:

“The basic formula to find your running FTP is:
(2,400 meter power × 2,400 meter time) − (1,200 meter power × 1,200 meter time) / (2,400 meter time − 1,200 meter time)”

But this site suggests that we do the 20-minute test, and then take 95% of that average power. My question is, which method is the best one for calculating rFTP? Thank you!