I'm using Coggan's suggestion of 106-120% of FTP and doing 4 sets of 5 minutes. It's hard but not quite hard enough to have me questioning whether I'm going to finish, which it seems like it should.
For level 4 I do 2 x20's at 91-110% of FTP. For Level 6 I do 5 sets of 2 min @ 121-150% of FTP. Both of those efforts seem to leave me at the right level of discomfort so I think FTP is correctly determined. It just makes me wonder if my Level 4 INT's should be longer - 2x10 or 3x7 instead of 4x5.
What say you?