Okay, I'm doing 80d so I took a size S because I was afraid that the size M is too large for the turn back (always my problem). In the end we see that it is slightly smaller because my chest returned but "pulls" a little on the elastic down, but I did not feel at all once worn. I will try perhaps the size M, but I fear that the elastic is too broad because, at this level there is nickel.
It's very convenient with racerback tank tops, you can not see the shoulder straps, you can not feel it, and it does not give the whole effect "without a bra."