check out this topic: Why I failed at TrainerRoad (I think)
it also includes my story with the ramp and 20 minute test: Why I failed at TrainerRoad (I think) - #68 by nvalphen
long story short, the ramp is not accurate for everybody, it can under or over estimate your FTP.
last addition to my story,
previous sub threshold workout (which was in fact an super threshold if it was scaled to a realistic FTP
did this one again last sunday, no backpedals, no struggle to keep the cadans, hr 5-8 beats lower (below threshold where it should be)
so I’m pretty sure that the 20 minute test is better for me, 
now I can actually spend 5x10 just under threshold without being killed,