I think most of the Forsaken are close in strength. I'd propose the following...

90 - 100 strength : Ishamael, Demandred, Lanfear, Sammael, Rahvin, Aginor, Graendal, Sharina, Alivia.

80 - 90 strength : Semirhage, Mesaana, Nynaeve, Balthamel, Be'lal.

70 - 80 strength : Moghedien, Asmodean

I really don't think that strength has ever been a factor in Forsaken attitudes to one another. The Forsaken tend to look at the whole picture : strength, skill, reputation, previous triumphs, mental state. Semirhage tends to make people uneasy because she is such a horrendous torturer - she makes Lews Therin scared and there is no way she approaches him in power. Aginor has lots of strength but little else. Moghedien has little strength but can backstab anyone.

I see no problem with Semirhage having a strength of 85 and Lanfear 100. It would be an even duel if it came to it, and neither would underestimate the other. Semirhage certainly has talents that make her a dangerous adversary, and Moridin doesn't doubt she could kill Rand.

It's important to remember that any Forsaken who were weak or unskilled were killed in the War of Power. All of them can stand their ground in some way. Any large gap cannot exist between the Forsaken - Demandred would not ally with Mesaana if she was weak enough to crush... she has something to her name. They are not Aes Sedai - they don't defer to strength. They are confident in their own power and skill to challenge anyone. Semirhage had no problem going for Rand when her plan failed, and Graendal didn't consider strength when she took on Cyndane and Moghedien at the same time. They're mostly equal to one another, with a few differences.