have you read all of Coggan? It starts with a discussion of Lactate Threshold (LT) being the best predictor of success, and that it isn’t practical to measure lactate. Then he gives 6 practical ways to estimate ftp without doing lactate testing.
A 40-min TT is probably a better estimate of FTP than shorter protocols, like the 20-min test in Coggan/Allen’s book (using 95% of average power). If you like, go ahead and give it a reduction, say 98%. Unless your goal is to dismiss the observation that Coggan zones roughly map to that study, that 40-min TT should be treated as FTP (plus or minus a few percent).