Responding in story form.
Once I had to do Spencer +2 (6x3min @ 120% with 4 minute breaks), but it was too hot for the trainer. I went to a local hill which takes about 3 minutes, ish. The gradient varies quite a bit, with some steeper pitches followed by flatter sections, so it’s really hard to stick to a steady power.
So I just went up it hard, 6 times, keeping an eye on my HR so that I didn’t go out too hard first go. The repeats just took as long as they took.
Then when I got home, I analysed the ride, and the intensity factor for the six hill reps and rests in between was exactly the same as the intensity factor for the 6 vo2 intervals + rests in Spencer +2. Job done.
What I’m trying to say is, if you want accuracy and analysis, stick to the trainer. If you want to ride outdoors, then look at the indoor workout, look at the outdoor workout, and figure out what are the overall goals of this session, the overall intensity or work you need to aim for, and figure out how best you’re going to achieve that on the terrain available to you.