I was reading this blog post on training peaks: https://www.trainingpeaks.com/blog/tracking-aerobic-threshold-training-with-wko4/
It seems like zone 1/2 recovery rides/long-slow distance on the trainer should be perfect data for such a measurement. E.g. Flecter
This kind of tracking is important for me because I do 10+ hour races. Ultimately, I want to maximize my power at VT1/aerobic threshold for these long efforts, so that means I want to track my efficiency factor(EF) to make sure I’m getting the most out of my aerobic work and that I don’t lose any substantial aerobic fitness while I put in some high intensity in the lead up to my A races.