RJ said 37.5% of female channelers fall below the level required to qualify for the Shawl.

It is clearly stressed that Daigian passed this test by a hairsbreadth.

Hence, Daigian clearly falls on this boundary. Put differently, you cannot be weaker than Daigian and still pass the test.

So therefore we know to a very high degree of accuracy that Daigian falls on this 37.5% boundary, or as close to it as a living channeler can apparently be.

From that point we can extrapolate the rest, as we know the number of SD's that cover the percentage of the population between 37.5% and 50%. It is 0.32SD exactly, if I recall correctly. One third of an SD, rounded for quick reference purposes.

The rest of the formula falls into place then, proving that a normal distribution is impossible, given the strength facts we are given in the series.