It seems clear from Moghedien's comments, and from a host of other references, that the blocked Nynaeve in Book 4 was still FAR from her full strength. In fact, a strength of only two thirds her full strength is quite possible at this point.

The significance of this single fact is that it puts Moghedien at only two thirds of Semirhage's strength. And this means that it is possible for a vastly weaker Forsaken to survive in the company of much stronger enemies.

And this in turn means that it is not a prerequisite for Forsaken longevity that they are all within 5% or 10% of the maximum strength.

It is obvious that if Nynaeve is not even at her full strength in Winter's Heart yet, then she must have been very far removed from her full strength when she matched Moghedien back in Shadow Rising.

So it seems virtually certain that Moghedien is far, far weaker than Semirhage, and therefore inconceivably weaker than Lanfear.

In my mind, this gives us an absolute low extreme and an absolute high extreme for the female Forsaken strength range.

And therefore it is not unreasonable at all to suggest that the remaining females are spread more or less evenly between these two extremes.

Meaning that they aren't all grouped just 5% below Lanfear's strength, with only Moghedien lying about 30% or 40% lower.

Instead, you probably have Moghdien at about 65%, Mesaana at around 75%, Semirhage and a full strength Nynaeve at around 85%, Graendal at 90%, Cyndane at 95% and Lanfear at 100%.

Remember, if Nynaeve was only at say 70% of her full strength when she matched Moghedien, that would make Moghedien 70% as strong as Semirhage. But since Semirhage is not at the highest possible level for a female, that would probably mean that Moghedien is only around 65% as strong as Lanfear.

I find this the most likely distribution of female strength.