The evidence can best be pieced together as an overall picture, like a puzzle, that is reconstructed from a multitude of bits and pieces spread across the books.

The logic loosely goes something like this:

The weakest Aes Sedai - Dagian - is approximately one third the strength of the strongest Third Age Aes Sedai - Moiraine, Lelaine, Romanda, Siuan and Elaida.

So if you make Dagian a 10 (to use a round figure), then Moiraine would be a 30.

Egwene is stronger than Amys and Melaine combined. There are two contradictory quotes on Amys strength, but the most realistic one puts her about 1 level below Moiraine, but still stronger than most modern Aes Sedai.

Melaine is weaker than Amys, but still above average for an Aes Sedai.

So, a strength of 50-55 would be accurate for Egwene in this context.

Egwene herself makes it clear that there is a big gulf between herself and Moghedien. She says she stands no chance against Moghedien. That's from her own mouth.

Moghedien is the weakest of the Forsaken, and Nynaeve - still FAR from her maximum potential was already able to equal her in Book 4. Nynaeve easily still had 25-30% of her strength still to GROW at the time that she equaled Moghedien. I don't think anyone can really dispute that. Especially since even as late as Book 8, Nynaeve said she STILL had not reached her full potential strength. This was MUCH later than when she matched Moghedien in Book 4.

So, if Egwene is sitting at around 50, then Moghedien has to be so much stronger than her that there can be no hope of Egwene standing a chance against her. That means Moghedien has to be at around 65 at the weakest.

If Nynaeve is 25% stronger than that - or even just 20% stronger than that, it would take her well into the 80's on this scale.

At the same time, we know that Alivia is CONSIDERABLY stronger than Nynaeve. We also know that Graendal is stronger than Nynaeve, that Cyndane is stronger than Graendal, and that Lanfear is stronger than Cyndane.

So there has to be enough room above Nynaeve to adequately incorporate these strength levels as well.

In a nutshell, this is the "established evidence" that I was referring to, which fits nicely with Elayne (at 50) needing to almost double her strength to get to the level of a powerful female Forsaken (at 90).

If she only added 50% to it, she would be at roughly Mesaana's level (75). If she almost doubled it, she would be at around Cyndane's level (90).