Full disclosure: I'm a good friend of Terez, so we've discussed the matters a bit, but she didn't discuss with me in very specific details the strength matters much more than she did tell you.

A few things. That she told me, she hasn't seen the bell curve graphs, the formulas and all. Harriet is probably still using those files and lists. T. could't give them to us anyway if she had gotten them, as they could turn out in the Encyclopedia for all we know.

However, RJ apparently often "mused" to himself in notes, and "reasoned aloud", so his initial distribution of channelers and their strengths among Aiel had that and some expanded calculations, and relations to the bell curve. Apparently it confirms all he said about % and such. The WO simply train a greater range of the bell curve than the AS would, probably closer to what the AOLers did. Thus this would confirm its function was the distribution of all abilities, from Lanfear down to a level so low we've never seen anyone from it.

The only evidence that Moiraine could be at the bottom level of RJ's list (but it's more probably "very near" the bottom as her ability isn't inexistant), is that

1) She has the lowest numbers found so far
2) She's below a level under which a woman should not sense the source clearly enough to be trained to embrace it. But she's trained, so all she needs is an angreal to be able to channel as efficiently as before.

The bell curve helped determine strength distribution and rarity and turn it into numbers of characters, for every group. Stats differed between groups and between men and women as several factors varied.

To reconstruct the system, there's a bit of speculation involved.

It appears that RJ initially ranked the sisters from level 1 (Moiraine) to level 21 (unknown sister, but perhaps someone seen in books 2-3-4.. or who didn't appear on screen before the system changed. I suspect it could be one of the Namelles).

Later on, it appears he might have cheated - he could as he had not specified yet who was at the bottom and his references to novices and numbers were still quite vague - and decided to lower the WT standards to have sisters as weak as Daigian (to make a point they were hardly useless, Daigian was weak but one of the finest White logicians?). The lowest level to be allowed to try for Accepted speculatively got lowered, and the same for the level to test for the shawl. It seems that he added 12 levels below the former weakest sister and put Daigian there. Most established women didn't change strength, but someone formerly "off the charts" like Morgase likely dropped a great deal.

This very possibly happened when he prepared to write TSR and determined the size of the Aiel clans and how many WO they had per sept, then how many could be channelers. The statistics showed him they had much greater probabilities of having strong women alive, and in some numbers, so he had to bring into play some "Forsaken class" women. Furthermore, he had decided Moiraine/Egwene would follow Rand to the Waste and meet the WO. That wasn't his first plan, apparently. Rand went alone and brought the Aiel back to take the Stone.

Anyway...that may well be when he decided he had better cover the whole bell curve and add the 12 +x levels above Moiraine to cover all the way up to Lanfear, as he now had to make comparisons between and keep track of women stronger than Moiraine other than Nyaneve + Egwene/Elayne, whose strengths he could keep straight in his head. He likely added all the levels below 21 that he needed to cover the curve all the way to someone having a null ability.

The +x system he must have introduced for the intuitively obvious reasons: the strength of sisters already appeared in a ton of notes, if he renumbered things, the risks of missing one # in a file he used and using a wrong number as he wrote were too high. He had to keep those numbers as they were, and his system to calculate behaviors between AS was all conceived, so again he had to keep the ranges of strengths covered by each levels the same as they were (by all internal evidence, the range covered by one step is probably constant, ie: RJ judged the logical behavior of every sister by simply counting how many steps separated them. x steps = deference, y steps = obedience etc. Cadsuane is x steps from Merise, and x steps from Nynaeve etc. He used it that way even for +( characters like Elayne and Cadsuane vs. non plus x ones, so it looks somewhat doubtful the range covered by a step increases the stronger women get, otherwise he simply had to logically reflect that growth curve in the numbering of his levels to still be able to make comparisons at a glance. That he didn't do that but used linear numbering like the first system suggests, but I guess doesn't prove yet, it's linear.

So on the one hand RJ added (+ levels to maintain his existing sisters at the levels they appeared in the notes. He also introduced a second number for each woman, also linear (+1 per step), to rank everyone from Lanfear 1+12) all the way to a speculative bottom level (70? 100? It's impossible to be sure. Moiraine's the lowest known).

The "old style" number let him compare anyone to an Aes Sedai, the new numbering let him know where each woman stood globally.

The new numbers corresponded in turn to (greatly rounded up) life expectancy ranges. Those LER# corresponded in turn to an aging factor (%) and the start of slowing. This all determined mathematically the physical appearance of sisters, WO, Forsaken women after RJ compensated those numbers for the Binder effect, for the harshness of the Waste or life at sea, etc.

The comparisons between aging factors might reveal if the scale is linear or follows some curve or is somewhat exponential, but maybe it won't. From what I've heard there's really not a great deal of difference between the life expectancy of people on the 12 (+ levels or even between them vs. Elaida level sisters, while there is about 3 times that gap between Elaida and Daigian. I don't know what you math-heads can make out of just that, though...