[skidlids]

Trouble is paul unless you are prepaired to experiment and invest in several sets of springs to find what will work for you on each bike, you are better asking somebody that has done it. people like Maxton, Dave Parkinson, KAIS, K-Tech, etc i always used Justin at WPS until he got snapped up by Red Bull, but when it came to sorting one off specials I learnt to do it myself.
Tonio currenlty runs the standard progressive spring that he doesn't seem to be happy with and therefore wants a change. He has been recommended a couple of different options. I know his budget for DD is tight and therefore he doesn't want to waste money on the wrong springs so he is trying to understand how the options differ.
personnally I try not to race with anything other than single rate fork springs hence me recommending the Ohlins Option, No doubt these springs can be supplied in different rates and that will be down to how you like your bike setup (front/rear weight bias) and your body weight. As Tonio looks to be about the same weight as me I would suggest trying 0.85Kg/mm but its his money and therefore his choice.

I agree the Shear Modulas of the steels used to form springs varies over a range and I settled on a fixed value between the upper and lower figures to use in my equations and therefore my calculations have always been an approximation close to what is required, but as my spring supplier provides me with rear shock springs in 25lb/In steps then I usually go for the one closest to my calculation and at just over £15 a sping having to buy a second after testing the first and deciding i need slightly harder or softer is no big deal after spending £6k to £8k on building a one off special or a personalised Harris Magnum. Its always harder when you start with thin air and I have learnt to always go for a shocck with as long as stroke as can fit in the gap and get the linkage to go through a curve as linear as possible.
The linkage and shock that Harris provided for my Magnum 4 was awful, the ratio went from 2:1 to 2.5:1 and then the shock didn't have enough Stroke to cope with it. As mine was one of the first Magnum 4s later ones came with a Ohlins shock tailored to suit them and a rocker that only gave a ratio range of 2:1 to 2.1:1, Dallas had one of these later ones and it handled a lot better at speed than the early one I had, although over time i did get mine sorted