I was always under the impression that on a ramp test, your cadence should remain broadly the same throughout. The moment you find yourself drifting significantly lower then stop rather than continuing to grind it out.
The advice from TR for their ramp test protocol is to go for as long as you can until you cannot maintain target power: "go until failure".
https://www.trainerroad.com/cycling/workouts/385142"Stay seated for the entire test, and don't worry about any specific cadence. Simply turn the pedals until you can't reach the goal power any longer. The test is complete as soon as you are unable to pedal or your watts begin dropping, and there is no benefit to continuing once your watts start to fall."NB in Erg mode, where the trainer forces you to match target power, this means the test protocol is to go until your legs stop.
The number you get from this test is just an input for scaling training workouts, and while not perfect I've found it to be pretty good for that purpose, eg.:
- following my test in January subsequent "SSB II" workouts initially felt a bit too hard and to avoid too many failures I manually knocked the FTP number down a touch (before later raising it);
- following my test in May the "Climbing Road Race" workouts have felt spot on, no failures to-date, and perhaps I could raise it a touch.
I think if you do these workouts a lot and have been doing them for a long time you should get to know how they should feel, so could probably set your ftp number manually with good "accuracy", with any test protocol mainly acting as a periodic "sanity check" on the number you've set.