Before using TR, I followed another set of structured training plans where rest intervals were almost always at 75%. I was specifically training for a Gran Fondo / Century with much more steady state effort, compared to a crit that surges a lot (look at any of the TR race analysis videos to see that about 25% of the race is VO2Max or above, 25% coasting or active recovery, and the rest in the middle across all zones).
There is an argument to be made that if you can recover at 75% FTP (or so) then maybe your FTP is too low, but I’ll posit (bro-science with N=1 data points) that it depends on what you’re training for.
I seem better suited for longer, more steady state efforts so I prefer 70-75% for any recovery interval 3 minutes or longer. Much below that and I start to cool off too much and find getting back up for the next interval takes more demand than what it’s worth. Specifically, I like my recovery HR in the 130s (max is probably high 170s or low 180s). 70% puts my HR at 132ish, 75% at 137ish.
For shorter recoveries, especially 20/40s, 30/30s, and 40/20s I set the recovery to 50%.
I use Workout creator to edit all interval workouts to 1) change the warm up to a 15-minute ramp and 2) raise recovery intervals to 70-75% (3-minute or longer recoveries). Take my last two interval workouts, for example (see below). I altered the workout recoveries to 75% but ended up dialing them down to 70% due to increased temp and humidity in the basement. Also, I find that if I can do a bunch of workouts in a row recovering at 75% then it is in fact time for a ramp test.
Anyway, just my two cents.