Chris Carmichael of CTS (Carmichael Training Systems) has had a lot of clients, including of course LA, and a lot of time to investigate. His opinion, laid out in his Time Crunched book, is that there are so many variables involved, no matter how you do it, that the best thing is to take as little out of the athlete as possible when measuring. It's just a number, after all.

My interest piqued, I looked it up. Chris believes, after evaluating thousands of power files, that the CTS field test power result should be multiplied by .9 to get FTP, whereas Coggan as you mention, multiplies his 20 minute test by .95 to get FTP. I misremembered the spacing between the CTS test intervals: it's 10 minutes.