Here is my interpretation of the bell curve. The 21-level system is evenly distributed but the power levels on that curve are exponential.

The power of a channeller is given by l^1.21765144798273 (where l is the level on the 21-level system).* Here are a few channellers' powers given that equation (which I normalized between 0 and 1). This equation is based entirely on the following presumptions:

Egwene is exactly five times stronger that Daigan.
Egwene is level 15
Daigan is level 4

If you have better characters or corrections to these presumptions the comparison than let me know.

Lanfear 21: 1
Nynaeve 18: 0.8289
Egwene 15: 0.6638
Daigian 4: 0.1328
Morgase 1: 0.0245

*My math explained:
l^x = p (l = level, p = power)

5*4^x = 15^x (Daigan, a level 4, is 5 times weaker than Egwene, a level 15)
=log_4 5 / (log_4 15 - 1) (log base 4 here, I'm not sure what a good way of indicating that is)
=1.21765144798273

**All channellers levels were taken from http://13depository.blogspot.ca/search/label/Saidar%20strength . If you don't like the levels assigned, you can view the data it hde supplied link. If you want access to edit just give me your email address.