You might underperform on the ramp test. Ramp is pretty accurate for me, probably over my mFTP by 2% or so most times. But I train by my mFTP supported by field testing.
Best way to confirm is riding to MLSS by feel and holding it as long as you can. Once you do that a couple of times, you can periodically dial in your FTP without incurring a bunch of fatigue.
Remember FTP isn’t a precise measurement - it varies day to day and there are errors associated with measuring it. Getting it within 5W or so is good enough for most.