We are expressly told that Moghedien stood no chance against Lanfear.

We are expressly told that Cyndane has enough of a strength advantage over Graendal that she immediately notices it. This when Aes Sedai often can't tell who is the stronger channeler if two channelers are very close in strength, other than both channeling as much as they can to measure the difference.

We are also told by Graendal that Cyndane is "MUCH" stronger than Moghedien: "Where did the Spider find someone so MUCH stronger than her?"

We are also told that Lanfear is sufficiently stronger than Cyndane that the other Forsaken can clearly sense this difference at first glance.

To me, the important thing to remember is that unless you can effortlessly shield another channeler, any encounter will be dangerous, even if you are significantly stronger. Because weak balefire will kill as easily as strong balefire. And someone who is very fast and skilled with Compulsion can probably strike you much more quickly than your more powerful lightning bolt can, even if you exceed their strength significantly.

We have formed a good idea that you probably need to be between 50% and 70% stronger than someone to shield them relatively effortlessly if they are already holding the source. No one claims that Lanfear exceeds Moghedien by that margin.

But a situation of Moghedien at 65 and Lanfear at 95-99 is very possible, especially as we are told Moghedien had no hope of ever matching Lanfear in battle.

Moghedien is an extreme outlier, but given the 50% strength advantage rule for shielding other channelers if they're holding the source, a battle between Lanfear at 95 and Graendal at 85 would be very dangerous, especially if Graendal can strike faster with her speciality Compulsion weave than Lanfear can with her 15% more powerful general weaves.