There remains the fact except for Moghedien, the other female Forsaken have all impressed the third Agers with their strength. You'd think women used to Nynaeve wouldn't so impressionable if Nynaeve or Alivia are very close to the likes of Mesaana/Semirhage.

IMO this suggests a lot strength isn't linear, the further up and down the mean, the larger the gap between each level.

Keeping the strengths vague is a large part of what RJ intended, as you described. I think he also wanted to show it's not that relevant an issue among the Forsaken. It puts even more emphasis on the obsession for strength among AS.