I wouldn’t risk it, having experience the dark-side of training. I know you may be proud of your numbers and everything but it’s important to set the right variables for training to avoid over stressing and under stressing the body. Guessing on what is already a ballpark figure increases your chance of going down this road.
If you want to know your power for a particular duration then go and test it, otherwise you’re just guessing and this will lead to you chasing your tail in the future.
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Well technically, 20 is shorting from the true 60 
And the original protocol, unlike taking your best 8, 15, 18 etc…minute was designed to try and at least mimic 60 minute fatigue with the three 1 minute and the 5 minute all out effort prior to the 20. All designed as a proxy for LTHR as most don’t have resource to analyze blood lactate.
All pretty much whatever at this point. How ever you come up with a number to set zones that work for you to make you faster is the goal. If drilling it for 15 fresh works more power to you. Here’s the thinking behind the 20 from the man himself for those who are unfamiliar: