Depends on the engagement rpm that you want and is determined by peak torque of your engine, not the class.
FWIW, I like more weight and more spring tension.
Typical set up for unrestricted AKRA/NKA clone, WKA/IKF flathead and animal stock classes will be 1 bolt and red springs @ .240" - .225" for an engagement of 3800-4000 rpm. If you want lower engagement, add weights then adjust springs and tension (height) accordingly.