With Moiraine at a speculative 30 strength, two thirds of her strength is about 20.

If you're correct in your hypothesis, Lanfear lost less than 20 strength to the Finns.

This is fine, because I have always had Cyndane at only 1 level (5%)lower than Lanfear. In other words, Lanfear at 95, Cyndane at 90, Graendal at 85, Nynaeve at 80.

That puts Graendal at slightly stronger than Nynaeve (5%) but closer to Nynaeve than to Lanfear.

Given the stature of the Forsaken and the amount of strength drained from Moiraine, it doesn't really make sense that they would only have drained 1 or 2% from Lanfear. A decrease of at least 5% is a minimum in my view.

That's still only a quarter of what was drained from Moiraine.