For more information on what FTP actually is just go to the guy who invented the concept - Joe Friel. 3 seconds of googling for a definition comes up with:
Originally Posted by Joe Friel
Functional threshold power or pace (FTP) is the highest mean average power or pace you can maintain for one hour. That’s quite precise, clear and logical.
Source:
Joe Friel's Blog: Functional Threshold
If you think your FTP is 280 or 250 or 260....then show us the files where you have maintained a majority % of that number for over an hour. TT or race files help and are best as you'll be going hard enough.