This past Christmas eve - all full of cake and cookies, all the guests when home... I needed to burn some of the excess sugar out of my not so young, sorta fit diabetic body...

I loaded up zwift and rode at my estimated FTP for an hour, ended up doing a reverse split and was 20+w over my "FTP" for the last 10 min, and still had fuel in the tank, could have kept on going at the +20w level.

I think the answer to the OP's question - do you know your FTP? - after watching all the videos from all of the coaches and experts, after proving what they say in actual testing - the answer is no.

I think the best we can hope for is a range of +/- X%W (5%, 10%, 15%)?? And FTP timeframes for different people to actually be able to perform at said level - 40min to 1++ hour for different people.

I'm a diesel by nature - a grinder. I can maintain steady state efforts that are relatively high compared to my abilities. Higher intensity, short hard efforts don't end well.

My testing needs would be different from an explosive short effort type rider. They may crush the 20 min test, but fail completely at an hour long effort.

Best I can tell - FTP is a range that can vary from day to day, week to week. It's nothing more than a loose guideline to set training zones, or more importantly for guys like me - the average Joe - to set my power levels for long events.