See the comment from Andrew Coggan on the TT forum regardin FTP and the 'fitting' answer...
Here's Coggan's statement I've also provided a link to the full article if you're interested.
People make the mistake of focusing excessively on increasing their 20 min power, instead of focusing on what they really want to do, which is to...
increase their muscular metabolic fitness, i.e., their FTP.
Coggan TT forum
Here's a synopsis of the answer from an article (link attatched)...,
Three Ways to Improve Functional Threshold Power
JUNE 20, 2014 · BY NATE WILSON
Functional Threshold Power (FTP) measures the effort one can sustain for one hour and is the single most crucial factor in performance. Determine FTP then use these three workouts to increase it.
1. SUB-THRESHOLD/SWEET SPOT WORK
“Sweet Spot” = 83 to 97% of FTP. Steady state efforts, Sweet Spot is a balanced amount of intensity and volume. Ride without interruptions maintaining a steady effort.
Start with 3×8' at “sweet spot” with 4' easy. Build to 3×10' with 5' easy.
2. THRESHOLD CLIMBING WORK
To improve performance on long climbs do efforts at 100-110% of FTP on a sustained climb at quality power. Do them when fresh and piece out the intervals to stay under 60mins.
Start with 3×10' at 100-110% of FTP with 5' easy. Build to 3×15' at 100-105% of FTP with 8' easy.
The “gold standard” FTP workout is 2×20' at 100-105% of FTP with 10' easy .
3. FLAT LAND FTP WORK
FTP workouts should also be done on flats at 95+ rpm on the time trial bike.
https://www.trainingpeaks.com/blog/three-ways-to-improve-functional-threshold-power/