I think there is probably a 1-2% difference in Strength within the same level, with the highest level 13 being negligibly weaker than the lowest level 12 and the lowest level 13 being only a hair above the highest level 14. The strongest level 13 would be 3-5% stronger than the weakest level 14 .

Personally I think each level of Strength is about 3% lower than the level above ... which would roughly work out to Ishamael having about 20% more strength than Lanfear, and Lanfear having about 20% more than Egwene who in turn is about 15% above Moiraine etc...

So by the time you get down to the "Middling" Sisters in the mid-low 20's they are <50% of Lanfear and <40% of Ishamael. Daigian being about 25% of Lanfear and 20% of Ishamael .... which would still allow for the 13 weakest AS to be massively stronger than any Male but still be pretty weak individually. With someone like Moiraine's new strength being roughly 50% of Daigain's strength, so even 13 of the weakest women would be a little stronger than any man. Seems to fit the commentary from the books IMO.

If you assume Ishamael can channel 72 "Units of Power" (arbitrary based on there being 72 levels of strength) then you would have a scale along the lines of this:

72 Ishamael ++1
60 Lanfear 1
58 Cyndane 2
56 Graendal 3
53 Tamela 5
48 Egwene 8
42 Moiraine (old) 13
40 Sheriam 14
37 Verin 17
29 Elza 25
21 Siuan (new) 35
16 Daigian 45
11 Sorilea 57
8 Moiraine (new) 66

if Moiraine's angreal gives her a 6x increase that would make her as strong as Egwene, which seems to match the book's comment that with it she was actually stronger than her original strength.