I don’t know that answer, because it can change daily and also depends on a few things:
- the difference between the two numbers
- how close to zero it goes when testing? If you do some max efforts, see where the number ends up. If it’s frequently above zero, then you’re overstating, and if it’s below zero, you’re underestimating.
- Results after different duration and intensity.
In theory, you should be able to do the same kJ work across the duration timeline while power fluctuates up or down. Check it across multiple intensity levels and durations, and not only one duration max effort.
Some examples:
Assuming W’ is 20kJ and CP is 250W.
- 300W effort: 20,000 / (300-250) = 400 sec, or 6m40s
- 600W effort: 20,000 / (600-250) = 57 sec
- 270W effort: 20,000 / (270-250) = 1000 sec, or 16m40s
Edit: without taking fiber-type into account, the shorter or longer efforts could also skew the results. So there’s no fixed number, and does it really matter if it’s set exactly?