Either way, I agree that the better test is the HR that's closer to a lactate-threshold type training effort, which tends to be a much better number to guide power and HR training than using these oversimplified maxHR calculators.

That's the point of these 2 x 20minute FTP tests with a powermeter ( or trainer with speed sensor to convert speed to power). You will llikely hit close to your maxHR at the end, but it probably won't be an absolute max. Still you'll have a good number that is a reference for training and is more reliable than a maxHR number. The test still hurts just as bad as a maxHR test if done correctly though - that's an all out effort for 2 x 20, so you'll be drooling at the end if done right.

I'd just throw out that website in terms of its utility to you, honestly. If you're plugging in your numbers and the subjective feeling of zone4 (which it clearly states is race-pace) feels easy to you, it's clearly not working as it's supposed to. Read up on how to train with power or how to calculate HR zones through testing as I've described, and you can make a much better zone training assessment.