To work out what spring rate suits what rider weight you have to then factor in the shock linkage ratio, the weight of the bike, the weight distribution of the bike and the type of riding. Here are some pre worked out spring rates for the rear shock for road use on a Firestorm

So what is a Spring Rate? A Spring rate is the number of kg or Newton needed to move the spring by 1 mm (or inch if you use lb rating). To get a kg rate of a spring for a listed Newton force simply divide the Newton Force by Gravity So ideal spring for me Solo would be 170N so very happy with the 170-175 spring, So in this case the Acceleration is the Gravity which is 9.8 m/s. You can get the Newton force by multiplying the Mass and the Acceleration. Recommended Shock Spring Rate (for me 14 stone): 17.3 kg/mm (use closest available) I came about this trying to find out about my shock, i thought i had been sold a WP shock that was for a rider 170-175 kg in weight as that was what was stamped on the spring 170-175, thankfully Al pointed out to me that i was incorrect in thinking it was kg rating it was infact the Newton Force rating of the spring, so to anyone that has a got a shock second hand and doesn't know what spring is on it this may be of some use?

Ohlins spring chart pdf