power supply choke question

i'm looking to build a choke-regulated power supply for a small amp and was interested in the toroidal chokes made by Toroid Corporation of Maryland. i've calculated the critical inductance i need to avoid power supply resonance around the ripple frequency but i'm a little confused by the ratings of the inductors:

http://www.toroid.com/dcfilter.htm

"You can calculate the inductance of the choke as follows:
Divide Ws from column 7 (for the choke size you have selected) by the square of your rated current. Multiply the result by 2, and you have the inductance in Henry."

why is the inductance variable? does this have to do with the saturation of the core or something?

also, how do i calculate the minimum current needed to keep the supply rails at .9V RMS? yes yes i know i should just go look it up in one of my circuit books or something but i'm feeling lazy right now sorry. :p

thanks,
marc