I don't think there is an ideal turns to civ level ratio. It's more a matter of when do you want to rescue some people, build something in your settlement, scout areas and so on. I don't think I have advanced the metagame past 10 turns for the moment because I just explore all around and I don't feel like I need to advance in turns. I did build some stuff for everyone in my settlement (houses, workshops, ...), did some rescues and scout a lot with my people. For me, I pass turns when I want to improve my settlement after a long period of caving and exploration, just to change my mind.
For the moment, as you point out, passing turns don't have an effect on the difficulty and I don't think this will change. Or if it changes, I bet Arcen would do something to ensure the compatibility of your old world with these changes so you won't get stuck in an impossible situation.