I’m certainly not assuming or guessing as I have a pretty good feel for my power - a benefit of about two years of TR. Ramp, 8min and 20min tests are really just ways to estimate FTP - certainly a good way to do so for the sake of consistency. You could estimate it any number of ways providing that SS feels like SS and threshold feels like threshold, etc - TR workouts like Gray and Lamarck are also good ways to assess/confirm FTP.
I don’t do the Ramp as I find it doesn’t track well to FTP for me (as detailed in that thread I linked to) given my V02max challenges (I’m much better at steady state efforts around threshold). I don’t do it on the four week schedule as I find that my fitness gains don’t seem that follow that four week structure…or I just don’t feel like it!