You said that the Tower doesn't restrict women of higher strength. But the sample population of the Tower, ie. the thousand channelers we see, are skewed towards the center, with no one of very great strength or very great weakness. Which means Moiraine is not very far above the average of 50, and Egwene is clearly above it while Nynaeve and her ilk are far above it and closer to the upper end of the range.

It works really well if you make increments of 4.75. That gives us 21 levels, and you can place strong Aes Sedai like Anaiya or Myrelle or Sheriam at 50, and the list is quite good, in that case.