Originally Posted by
rpenmanparker
The trouble with OP's method is he has to run two tests to get one result. So it is a two-day process. Running the two tests in succession on the same day doesn't seem like a valid way to go about it if you want to use the independent 5 minute test and independent 20 minute test. Not to mention physical differences of the rider on those two days contributing to error in the method.
I've heard of similar testing methods to what the OP suggests, and you want to do both tests on the same day. The idea is to get a sense of how you fatigue during rides and factor that into your FTP. Doing both tests on different days negates that as you are fresh for both tests.