A steady upward trend in the Efficiency Factor metric as I progress across a season - looking for when it starts to stabilise which could indicate a need to change things up; extend endurance ride duration or add some different stimulus and/ or intensity.
Just after half way in my graph I start to plateau my EF as I come to the end of my base phase and a SST TTE block - I changed things up and moved into a Threshold TTE block, increasing EF further before moving on to a Vo2 block.
Its not an exact science and sometimes works out after the fact, but I do see some value in tracking it.