I think it depends what you write. I think Tom Kuegler probably has a higher than 50% read ratio overall, he writes relatively short pieces and they all flow effortlessly. I think for people who write the sort of mixed stuff i write, some funny, some serious, some writing tips - 50% is a good target to aim for. I'd be intrigued to know what people like Shannon Ashley, Jessica Wildfire and Umair have as their figures.

40% I just inherited from previous years of wisdom. One of those things that someone told me once when I was new on the platform and I've arbitrarily accepted as the benchmark for solid writing. I'd say 3 years later that's pretty true (with a few caveats for genre and length).

I put everything behind the paywall but I'll use friend-links to bypass it if someone is interested in my work and wants to read more but isn't a member. I did that fairly recently in the comments section. I can see why the writing for free thing might appeal but I think for me it'd be a waste of my time and effort. I'm enjoying short form as a way to signpost other articles - rather than as a way to get people to join Medium. As always, people should always do what works for them. :o)