SS offers different springs for Valks. None of them are progressively wound. You have to guess at what spring rate will work for you.
The procedure seemed pretty straight forward to me: Bike Type + Riding Style + Weight.
Try looking at Hyper-Pro springs. I just put a set in my 1800 Wing. Love them. They are the only spring that is progressively wound throughout the compression range. "Progressives" are a two stage spring as are most aftermarket springs that claim to be Progressively wound.
I never thought of Hyperpro. Thanks for the heads up! Something else to think about.
Sonic Springs
Hyperpro Springs