Good thoughts, there. I have a little spreadsheet which computes my NP, AP, IF and TSS for any number of work and rest periods, which I use to set my target power level during my interval workouts. Bascially, I experiment with different target powers, work durations and rest durations (assuming ~50%FT during the rests), to match the NP for the entire set (minus the last rest period) to my Mean Max NP curve from the Cycling Peaks software. For example, I may start by entering VO2 intervals at 115%FT at a 5:2.5 work ratio into the spreadsheet and increase the number of reps by one until I've broken out of my NP curve. If that happens after 3-4 intervals, and results in a TSS lower than I'm interested in (for this point in my training cycle), then I may go back and try a 112%FT interval or a 3 min rest duration and see if I can get 5-6 intervals before hitting the curve. As Andy touched on, I've grown pretty confident that I can physically accomplish any workout that falls under my NP curve (and even break it by a small amount on most days), so this gives me a little willpower to do for those last couple intervals even though I'm tired.