Firstly, just how much stronger do you think Graendal is than Nynaeve and Semirhage, if Someryn fits in between them on the strength scale?

Secondly, BS gave a rather strange answer to the Nynaeve strength ranking question.

He says that there are only 6 women stronger than Nynaeve on the strength ladder. He clearly cannot remember who the 6 are, but was very certain of the fact that there are exactly 6 women above her.

This leads me to believe that there was some significance attached to the fact that Nynaeve was the seventh strongest female of all. In other words, it stood out that she was in seventh position, but the names of those above her was not necessarily used in order to calculate her position.

To put it differently, her POSITION was known to BS, rather than which specific channelers are stronger than her.

This came through very clearly in the way he answered the question relating to Nynaeve's comparative strength to Semirhage.

BS did not know for a fact that Semirhage is not one of the women in the 6 places above Nynaeve. If he knew the identities of the 6, he could have said unequivocally that Nynaeve is stronger than Semirhage. But he did not say that. Instead, he kind of hesitated, mused on his general impression of the two women, and ventured a guess that he is PRETTY SURE that Nynaeve is stronger than Semirhage. He then tempered it by saying that they are very close.

Now, that is NOT the answer you give if you know that Semirhage is not one of the 6 names above Nynaeve. His ignorance - at the time of answering that question - of the exact identities of the 6 women above Nynaeve is highlighted by the fact that he placed Cyndane WAY below Nynaeve. If he knew the names of the 6, he would have known that Cyndane was one of them.

So in conclusion: BS just has a general impression that Semirhage and Nynaeve should be very close to one another in strength. At the time of answering that question, he did not know for a fact that Semirhage is not perhaps one of the 6 women ranked above Nynaeve.

So the 6 above Nynaeve could very well be:

Lanfear
Cyndane
Graendal
Alivia
Sharina
Semirhage

It does not by default have to include Someryn. And even if Semirhage proves not to be on that list, it could be Talaan, or even some as yet unrevealed female. I don't think the requirement for 6 women stronger than Nynaeve automatically places Someryn above Nynaeve.