My Vintage ring works perfectly (both the clear and black sides) with my Nexus 5, though the official Google red case too.
It's true that it takes a bit of practice and obviously, how straight it is around your finger makes a difference.
The heat map is a great idea; I love the suggestion, "please be awesome and share your sweet spot so we can improve the map" - I'm sure that helps.
It also works fine with my Nexus 7 which is not in a case.