% of FTP is fine for prescribing exercise plans, but probably wouldn’t be accurate enough for data collection for small group studies. There is too much variation between individuals in terms of LT relative to their VO2 capabilities. I think Harsh Reality - You OVER test on FTPs (support group) is a good illustration of that.
They tend to use other methods such as the difference at the LT and VO2peak. So they might look at Δ40%, or Δ50%, where the power output of the participant is 40% or 50% of the difference between LT and VO2peak. That way you can ensure the participants are undertaking comparable efforts.