Here’s an interesting article on the ramp test which seems to be written by the person who created it: Is the MAP (Ramp) Test a Valid Estimator of FTP? — CycleCoach | Personal cycling coach
tl;dr - “This is really where both Zwift and Trainer Road have got the details slightly wrong. Although they both spoke to me, and I made it clear, I explicitly stated that someone’s FTP was NOT 75% of MAP, I said it would fall into the range of 72 - 77% for the vast majority. However, the software as it is wants or needs a definitive, specific answer and they plumped with 75%.”
It’s said that your FTP will be higher outdoors than indoors for various reasons, and I think the ramp test gives me a number that reflects my outdoor FTP. I haven’t done a 20 minute test in ages but can’t imagine riding at 5% over my ramp test FTP number for 20 minutes, at least not on Zwift. Outdoors, uphill, in a race/event/group ride, on a cool day? Possibly… and much more likely, of course, if I took the 72% number instead of the 75% number.