Somthing that I picked up in Grassroots Motorsports.
"The approximate stiffness of a solid anti-rollbar can be calculated using this equation:
K = 1,178,000 x (D4/LA2)
K = bar rate in lbs./in
D4 = daimeter of the bar, to the 4th power, measured in inches
L = center length of the bar, measured in inches
A2 = lever arm length, squared, measured in inches
and 1,178,000 = the rigidity modulus consistant of spring steel."
Ok it is alot of calculations, but with this you could figure how much the rate would change if you had an adjustable bar or if you had an aftermarket anti rollbar adjuster like these
JIC magic adjustable end links