Derek, if you have the standard set up & not a fully sprung conversion then you have quite a number of choices. There were 3 types of springs originally, Comet, Rapide & side-car. You can mix & match to obtain what
spring rate suits you. You can also use dampers with varying rates of damping which will also affect your ride. Another alternative is the Thornton system but then it comes down to how much you ride the bike & how much you want to spend. I may be wrong here but I always thought the Petteford springs worked best on fully sprung machines, no doubt someone will put me right on that score. If you have a fully sprung bike then the weight of the rider is a far more important factor when selecting springs.