The Bell Curve often gets ignored with these comparisons. It's clear even women at Moiraine's level are not particularly common, in the series thus far we've seen what 20-30 women with more strength, if women of Moiraine strength were as common as 1 in 10 there would be more than 5 in 1000(ish) in the WT.

IMO even Moiraine is likely in the "flattening" area of the curve. Let's also not forget that there are several levels Sidious left out between Moiraine and Cadsuane so Egwene/Nyn/Forsaken levels are probably more rare still.

I'd say Verin/Alanna etc are more likely to be 1 in 10, and even they may be more rare since they are strong enough to Travel, which seems to separate the Average from the Strong.