Anti roll bar spring rates

["Jim Dincau" <jdincau@qnet.com>]

In order to improve straight line stability I want to put an anti roll bar 
on a 4 link, jacobs ladder, coil over suspended solid rear axle. The front 
end is a transverse leaf spring which is very stiff. How do I make a guess 
about the spring rate of the bar?
Jim in Palmdale