Idea seems to be working so far.
Week 1 was maybe 75% of TSS and into the Z4 workouts at baseline ftp value of 4w above my best Zwift 45min effort of 276w. So 280w ftp value for Zwift workouts.
Week 2 was 95% of TSS and same ftp value for the same workouts, but repeated a few of them part-way to get more time in zone
This week, I added 5w to the baseline ftp value and went back to week 1 Z4 duration, but keeping TSS at 95 to 100%. Now all at 285w Zwift ftp for the workouts.
Next week, try to extend duration on same workouts again but at the +5w value.
The one workout is under/over for 30min straight, averaging out to roughly 97% of ftp. So, being able to go up on that workout is promising as it's within shouting distance of the 45min bare minimum ftp duration I'm comfortable using with workout % calculations. Just adjust the workouts as I see fit based on the workout data. RPE and HR going down for same workout over time, add power or add duration.