Why don't they just make it simple, xx chromosome only? Skip the "feminine enough" part. What's the point of women's only sports anyway? Is it because xy humans would almost always win and they want to encourage xx participation?
While uncommon, XY females do exist. There's Swyer syndrome, and Androgen Insensitivity, among others. Testosterone levels will usually be within normal ranges for XX females and they don't go through a male puberty. Should they be denied the right to participate?
What percentage though? It's a game, so the whole point is to make the playing field as even as possible. It sucks if you don't have the qualifying DNA to compete in the women's league and you really want to. Perhaps if there's enough demand they could start up a new league. For those with non typical sex chromosomes.