There is no reason to believe Daigian is .3SD below the Mean. You have made it a non-linear BC so she can be below the Mean and still stronger than 32.5% of other Channelers. It doesn't suddenly become linear when it suits your argument. The only thing we can say for certain at this point is that Morgase and Lanfear are the same number of SDs from the Mean.

Daigian could well be lower than the 62.5% number RJ stated as we know the Tower has been weakening for centuries. It's possible they let many women through who would have been out out in earlier years. Personally I think she is about a 16 on the 100 pt scale. Makes Lanfear about 6.5x stronger than Daigian, Verin is about 2x, Moiraine around 3x, Cadsuane 4x, Egwene 5x and Nynaeve 6x ... If the Mean is around 25 it all fits together nicely with the quotes from both the author and the text.