I'd say it really depends on how steady you are at keeping power applied while outside compared to inside.
Assuming you ride non-stop inside, and you probably aren't pedaling 10-20% of the time outside (just guessing), the .75 conversion earlier is probably close enough.
But one thing I've noticed (and it probably doesn't matter in training for a century) this year after a winter where I spent a lot more time than I wanted inside: the workouts are different, at least for me. I get a better pure aerobic workout inside where the lack of breaks means aerobic power generation is really developed. But there are also no power spikes, so my high-level anaerobic power seems to be really down. No, it doesn't just seem to be down - it IS down.
So while trainer time is better for my aerobic fitness, it doesn't do much for pure power.
And yes, I know it's possible to do z5 and z6 workouts on a trainer - I do it quite often. But there's no way to do the large power spikes on a trainer that can be done outdoors.