FWIW, I have come around to the concept of regular testing to define my Critical Power Curve (ala Monod) rather than just my FT. It gets a bit complex, but what drives my new approach is the recognition that two distinctly different capacities determine race performance (both mass start and TT events). These are of course aerobic and anaerobic capacities. I'm ignoring neuromuscular capacity for the moment, not because it is not important in determining race performance but because I don't really manage neuromuscular capacity during a race. It is what it is, but I use it once and when I use it I don't have the luxury of managing its use - I have to go with the situation at hand as the sprint unfolds. But, I can (and do) manage my use of the other two capacities during a race to my advantage. In fact, soon I will be able to manage these resources with extreme precision and real-time.
I'm still tinkering with the specific power/duration combinations to use for my regular testing, but at the moment I am planning to use 3, 6 and 30 mins. I think the 3 & 6 min durations capture the short end of the spectrum and the 30min test is a good duration for the long end (although the 30min test is the most flexible and can be anywhere from ~15-60 mins, depending on circumstances). Then, I have a complete CP curve and can predict with reasonable accuracy my MP at any duration (although it is less reliable for very short (e.g., <1min) and long (e.g., >180mins) durations. One of the advantages of this approach is that I don't have to accurately guess my MP for the duration. I pick a target power and then ride at that power (constant power) until I can't hold the power any longer no matter how hard I try. I then record the actual duration. So, it doesn't really matter whether I hold the 180sec test for exactly 180 seconds. I use the exact duration in the CP curve, whatever it is. The same holds for the other durations. Rest and motivation do matter, but I'm thinking I can do all three of these tests on the same ride in a progressive sequence (short to long) with sufficient recovery after each of the first two tests (e.g., recovery duration = 3x high-intensity effort duration).