I assume that this is some effort to see whether Graendal's statement on women of exceedingly rare strength applies to Nynaeve. Well, I don't think it does.

Although Nynaeve is one of the strongest women in the world, there are actually quote a few women with confirmed greater strength - Lanfear, Alivia and Sharina. Talaan matches her. Someryn, Graendal and Cyndane are all possible contenders to exceed her. RJ tends to have one Forsaken strength channeler from each race/faction, with one channeler of Nynaeve's strength in perhaps 1000 channelers i.e.

Aiel - Someryn
Tower - Nynaeve
Salidar - Sharina
Seafolk - Talaan
Seanchan - Alivia

I think the strength levels work in powers of ten...
- 1 in 10 - Moiraine strength
- 1 in 100 - Egwene strength
- 1 in 1000 - Nynaeve strength
- 1 in 10 000 - Graendal strength
- 1 in 100 000 - Lanfear strength

There is no answer to the question except to ask whether 1 in 1000 means rare, or very rare, or exceedingly rare to you. In context, Graendal's statement to me means that she met perhaps one or two women in her entire lifespan of over 400 years who exceeded her in strength, and thus Cyndane was exceedingly rare.

Your question is totally subjective, so it's up to the reader to decide.