Yeah, that’s the tricky thing with these slightly above FTP intervals. Even if you have a correct figure when you start a block, your training plan may actually work and your FTP increases over time 
You’d also have to suffer much longer, and I think ultimately, if the purpose is to push you to the VO2max limit, you know the point where you start panting and count seconds rather than minutes, then 120 % seems like the better way to go.