On a ride by ride basis I would just focus on keeping your power in Z2 at all costs, don’t worry about matching an indoor workout exactly for time in zone. Then on a weekly basis look at your total time in zones and make sure you are hitting your marks for time in Z2. No matter what, trainer work will hit the zones more precisely, for example on a 12hr all indoor week I got almost 10hrs in Z2 compared to some 12-13 hour outdoor weeks I would have only 6-7hrs in Z2.
Outdoor Weeks:
Indoor week:
