Originally Posted by GhostRider62

What is probably more useful is mapping your entire power duration(PD) curve from 5 seconds to 40-60 minutes. I don't trust Garmin for these types of tasks.

If the PD curve tells you you can ride 10 minutes at say 300 watts and you have a 10 minute climb, you know not to try it at 330 watts.

GoldenCheetah, TrainingPeaks, XERT, ZWIFTPower, and others will do this mapping/modeling for you. GC is free.

FTP estimates based upon 20 minute power is a bit of a crapshoot. FTP might represent 92% of the 20 minute power for some and 96% for others. This 4% difference could matter a lot within the context of a time crunched training plan that does a lot of intensity. In short, it could mean the difference between being able to do the program and the program doin you.